What resolution do you want to play at? For 1280x1024 a plain 6800 is fine, for 1024x768 a 6600GT is fine.
If you don't need 1600x1200 you could get one of those new from a safer place than eBay, then upgrade the motherboard later. The drawback is that you'd then need an AGP motherboard.
If you upgrade the motherboard now, you'd need AGP to use your current card, which again forces you to buy an AGP replacement video card later.
The best option might be to keep saving until you have enough for both a PCI-E motherboard and video card.