For one thing a FX5600 won't play those games and those cards don't come in a PCI express version. I have no idea what your vendor was talking about, never heard that one before. And your budget is what? $2500 or $250? If the first one, then you could build a really awesome computer to last you a few years, if only $250, a video card could cost you that much.
If you buy a newer motherboard, you'll have to buy a new processor, new ram, and a better power supply.
You do not have to get a board with DDR2, as DDR1 isn't on the way out, yet. Also, many gamers still use motherboards that support an AGP slot, with a decent AGP video card.
Tell me the correct budget and we'll go from there.