The difference is huge!
The X800pro is a better card, end of story.
The X800pro Vs X700pro:
Wins
X800pro has 256bit memory bus wide (the more the better the quality handle and speed)
X800pro has 12 pipelines over the 8 that X700pro has
X800pro has a better Memory Bandwidth …ALOT better.
X800pro has a better Fill Rate ...ALOT better.
X800pro runs on higher clock rates.
X800pro is
AGP x8 while X700pro is for
PCI EXPRESS ONLY!

Which is why it doesn't worth to buy.
(There are more to the good stuff but that summons it)
Lose
X700pro sucks against X800pro, You don't compare the mid range against the high range cards, ergo the numbers 700/800
X700pro manufactured in a different Architecture (RV410) and in different Manufacturing Process (0.11 Micron)
X700pro is for
PCI EXPRESS ONLY
Bottom line, get a X800PRO and you'll thank me :thumbup:
And yes, your PC is fine for the X800pro.
It doesn't even support PCI EXPRESS--> Take the X800pro !
P.s.
If you are going to think about GeForce 6800
Do not buy the LE version 