My existing hardware

2.00 gigahertz AMD Athlon 64
128 kilobyte primary memory cache
512 kilobyte secondary memory cache
Board: MS-7030
Bus Clock: 201 megahertz
BIOS: Phoenix Technologies, LTD 6.00 PG 07/28/2004
2048 Megabytes Installed Memory
NVIDIA GeForce 7800 GS [Display adapter]
Creative SB Live! Value

I have a Romtec Trios switch box http://www.hothardware.com/article [...] FSelector/
which allows me to startup one of three operating systems, Vista, XP pro, Ubuntu.
I use the XP for playing games and it is starting to struggle on some of the new games. Medal of Honor Airborne being one, I know thats probably not new to you gamers but I usually wait till the prices drop before I buy. I know that a complete upgrade is pending but I thought I could put it off if I just upgraded the Graphics Card. The question I have is if I buy a agp card that is Directx 10 capable for the Vista experience will it still work in the XP system which I don't believe can handle Directx 10. Since it is a temporay fix I really don't want to spend more then $150 for the card so if any suggestions on a card that would work well with my existing hardware would be appreciated.

Yes. You'd have full backward compatibility with a DX10 card... although everyone here is probably going to tell you it's time for a complete system upgrade.

Thanks for both the response and the warning and I will look forward to see what they have to say.

Yeah your best bet is the 3850 AGP, but it is rather expensive compared to the PCI-e versions. Last I checked it cost similar to an 8800GTS 512.

Any of these new DX10 cards would be fantastic DX9 cards under XP.