Upgrading Dells is rarely worth the effort, IMHO.
XP is no longer supported - either go with a linux OS or upgrade to 7.

Better yet, you can get an off lease Optiplex 760 for less than 300$, with 7 already on it. Displayport for HDMI, etc.
But whatever floats your boat!

if this is your machine
http://www.cnet.com/products/dell-dimension-2300-p4-2-4-ghz-256-mb-40-gb-crt-17/specs/

literally the only thing you might reuse is the case.
and I am not even sure you can do that with a dell. I remember in the past both dell and hp used some weird proprietary form factor, so a standard aftermarket board wouldn't bolt up.

the power supply is too small.
the main HD is IDE and also too small.
the optical drive might be reusable, but again, you are going PATA instead of SATA which is the common standard now.

you might save 30 bucks starting with that case assuming it has the hold downs for a standard ATX or micro ATX board.

You're on the bad side of the supply/demand curve. Components very pricey for what you're wanting to do. (I'll bet you mean 1Gb, but that's just me being nitpicky. )

The last time I tried to upgrade a CPU, it didn't go well. The OS has to recognize it, the heatsink has to come off & go back on without breaking and you need new compound on it, to boot. IF you're doing it a 'get my hands dirty' type practice, ok. But remember, you can spend 3x the cost of a new machine and still end up with a boat anchor.

Good luck, regardless!

You might try a local computer recycling outfit. They probably will have newer machines you could put that HD in, (and with a new OS) do a better faster job. Then give the old machine to them. (Once you have the newer one working.)

Tigerdirect and Newegg both list only 1 processor for a socket 478 cpu - and they both want $180 or more for it.

Google shopping shows that motherboard socket was current in 2003. I'd wonder how long it would hold up.

Before I spent that kind of money on a 10 year old computer, I'd look at Tigerdirect's refurbished computers.

Optiplex 760
$230 w/o warranty.

YMMV

depends on what your goal is.

if you just wanted to tinker to see if you can "upgrade" a machine. or rather take it apart and put it back together, then don't spend any money, except maybe on some thermal paste.

I would just gut the box and then see if you can put it back together to where it will run.
thermal paste is pretty much the only expendable in that.

if you need to have a faster machine, that optiplex linked above is way more machine than you could upgrade the old one to.

if you want to build one from scratch, then that can be done for as little as 400 bucks depending on the sales you find, and how long you want to be able to upgrade that base machine.
I have built many sub 500 dollar systems over the years that are still up and running. in some cases including the os, due to student discounts. in others, reusing the old hd with the os already installed.

If you are going to connect out to the net, you'll want to upgrade the O/S, or keep no personally identifiable info on the Dell.
And if a 10 year old computer is "faster" I shudder to think what your other system is like. Win 3.1?

Regardless, keep us posted, so we can tease help as needed.

a new cd/dvd burner can be had for around 20 bucks.
I think you can even get a blueray burner for around 50 now.

at this point however, if they are all cd burners, it depends on what connection it needs.
newer machines are all SATA, older are all PATA.
while you can buy conversion cards to let you use either in either machine type, it is a waste of money for an optical drive.
paying 15 bucks to convert, or 20 bucks for a new one.

what do you mean by back up?
like act as an extra machine incase one goes down?
or back up as in storage of images?

what are the specs on the laptops?

I have three socket 478 based computers in my workshop right now.  All have some variety of the no longer supported windows xp on them.  The most "powerful" one has a Pentium 4 2.8ghz.  The weakest is a 1.8? P4.

I spent this evening relocating the best parts of the 3 into 1 modern micro atx case I had taking up space.  All three orgininal cases are well built, but very poorly ventilated.  After getting it reassembled, I ran the thing to see if it was still funtional and was greeted with a arthritic old dog of a PC. Its great that it still runs, but not worth the trouble, other than I like building things.

I spent half an hour trying to get a ubuntu liveCD (dvd) to run, and it just couldn't be bothered to read it.  I then tried a Lubuntu liveCD (cd), and BAM.  I suddenly had a usable PC.  I just plugged in an usb wifi card and was easily on the net.

Unless you are going to put a thin linux based OS on it, there is NO point in maintaining that old tower.  Also don't change the motherboard as that will constitute a "New Computer" and your existing windows key will be invalid.  Furthermore, you cannot buy the retail windows 7 anymore and the "OEM" copys still being sold are one use only versions, so you may only transfer your os/key if the new motherboard fails.  You probably already know that the end of support for XP means that any computer with that os should not be connected to the web or it will be an easy target.

Just another example of how bloated windows is, I have a 4 year old netbook that has never performed all that well with it's "Starter" OS.  I test drove Ubuntu on it this week and had similarly noticeable performance jump. I have long since removed as much bloatware from it and run web browsers with minimal plug-ins, but it is still slow on win-starter.

I fully recommend buying a reconditioned, complete computer before sinking time and money into a computer that age.  That is, unless you are a tinkerer and a glutton for punishment, like me

Terrific! Hope it works out.

As others have said, not much can be done with an 11 year old pc.  I have a Dimension 2400 running Ubuntu, but it's not a regular use pc.