If I were in your situation, I would get an SLI capable mb with pci express, and spend less on your video card. When you do have the money, get a 2nd card and run them in SLI.
Pros of doing it this way:
You will have current technology cpu and mb
You will have current technology vid card
You will not be stuck with a video card that will not do SLI (unless you get a crossfire mb and an identical card) and AGP is dead.
Potential for greater performance when you get a 2nd card and run in SLI
Cons:
Initial cash outlay will be grater for a smaller gaming performance boost.
You have to buy stuff at best buy (newegg - good prices and no sales tax)