IF the AGP bus will only see 256MB, then it will only fill 256MB of your VRAM, thus making the more powerful card a waste. A better card would be a 7800GS or a X1800 series card. Or better yet, a PCI-E motherboard (nForce 4 say?) and a midrange PCI-E card.
Cheers
Cameron
The AGP Aperture of 256 in no way hinders a 512mb card.
That number is the overflow reserved by the system in the event the memory on the video card is overtaxed. It will not in any way slow a system down in games unless it is set too low. With 64-128mb cards an apeture value of 128 is fine. With a 256-512mb cards an apeture value of 256 is fine. Where the 512mb Aperture comes into play is in real world 3D art rendering with professional 3D ATi and Matrox cards, otherwise 256mb is fine.
The tests run that show a higher Aperture has better value were based on benchmark test and were never proven to hold water in real world use. I am here to tell you its bull-chit too. Although I would not run a 512 card on a 128mb aperture, that can be done too.
The issue with his system is the power supply and the memory bus for the x1950pro AGP. The x1950 will most likely draw more power than the PSU can deliver but it is not possible to know that for sure without seeing the rail specifications.
The bandwidth on the card may also exceed the CPU/memoy bus abilities. I have not looked it up so again, I can not be sure of that.
I would say without looking everything up a 7800GS 256 would be a better choice however it also has PSU needs and I would be sure the current PSU can deliver at least 20A on ONE 12v rail not including the rest of the system before buying the card.
The 9800pro is also power hungry but no where near as hungry as the 7800GS