Im sorry for the late reply DBonniwell.
There is no reason why an ATI video card must be used, you can use Nvidia aswell (which is the other top brand).
Looking at the spec sheet
here your motherboard has a PCI Express x16 slot which is good and will support a good graphics card.
This also depends on your needs, do you require a card for hardcore gaming?
I recommend this for 250, i dont really recommend going for anything higher priced or specifications as DX10 cards are reducing in price and Vista is almost out. Will you be upgrading to Vista? This card is very good for the money.
Newegg.com - GIGABYTE GeForce 7900GT GV-NX79T256DP-RH-ED Video Card - Retail
ATi option:
Newegg.com - SAPPHIRE Radeon X1950XT 100186L HDCP Video Card - Retail
Let me know if these cards are ok or if you have any other points to make before ordering.