Went to a gun show this weekend. Talked to a man who assembles ar15s. Told me to stay away from cast receivers.  

What difference does it make in real world shooting. (Target shooting but not competition)

Also, He would use rock river receiver and other quality parts. about $800.00

Does this sound reasonable? Can i do better?

All I can tell you is forged components are generally stronger than cast.  How much I can't tell you.

I have a few of each and unless you plan on doing bayonet drills on dock bumpers for 8 hrs a day, either will work for a shooter.

Quoted: Also, He would use rock river receiver and other quality parts. about $800.00 Does this sound reasonable? Can i do better?

DO NOT BUY!  You can get an RRA lower with a national match trigger for $290, then you can buy any upper you like and throw it on.  $510 buys you something really nice as well.  Look here:
www.cmmginc.com/inventory_a/uppers_tbl.html

My personal choice would be a 20" ss fluted heavy barrel A3 upper.  These are just a couple for you to look at.  I've purchased DPMS uppers for $350 and $400 and they were match barrels.  Combined with a $300 lower, you have a $600-700 gun.  Ebay is a good source for complete uppers as well.  If someone's upper doesn't sell, contact them after the auction and negotiate.  If you need help, IM me.

Chances are, you'll never have a problem with a cast receiver.  As stated above, most people don't give their rifles enough abuse to break a cast receiver, BUT their is always Murphy's Law so the decision is up to you.