FS 2004 is awesome. No comparison in the two. I have and love FS 2004.
I got Lock On: Modern Air Combat for Christmas, and it's been kicking my ass, and that of my PC (Athlon XP 1900, 1.24Gb memory, 128 Mb GefoRCE fX5600 Video Card). The graphics are so intensive and realistic that during low level flight (the closest to real I've seen) with explosions, I get down to 3 or 4 frames per second. Kinda sucks, but happens to all the systems, even those with more "OOMPH" than mine. I highly Recommend this sim if you like to shoot at things. Just be prepared to die alot.
Flyable aircraft are:
A-10 (Rocks!)
F-15C
Sukhoi Su-33
Su-27
Su-25
Mig-29A
I now have to upgrade my processor (probably to a P4 3.2 GHz HT and more memory, about 3 gHz) and as I read it, the sim will still push hell out of the frame rate. Damn Them.
It's awesome.
SG