If your current mainboard already has a PCI-e slot, I'd enter the BIOS, disable the onboard video, and slap the 7800 in....then enjoy!
(In gaming at decent resolutions, the video card is nearly always the bottleneck anyway, and a 3500+ does just fine in all games...even when I did OC mine to 2.5G, there was no real noticeable boost in average framerates)
I have an Abit AN8-SLi and it is great, it offers so many overclocking abilities and works like a charm. I am about to build a computer for one of my friends and i am going to get him the exact same kind i hae cus they are soo good, you can overclock like a mofo even on stock cooling. 8) Overclocking Face (straining for the most) :x