Or you can also move to AM2 for around $300 for board, CPU and RAM
Don't feel like linking right now but you can get all this stuff on newegg
CPU- x2 4600 - $115 I beleive
Board- forgot the board but there's 2 or 3 that combo with the 4600 bring it to either $225 or 235 I beleive
RAM- then there's some red g.skill ddr2 800 for $90 I beleive. There's cheaper 2gb kits that are ddr2 800 also, go down to about $70 I think.
So yea, the boards both have SLI I'm perty sure, there's also an ECS AM2 board with crossfire if you wanna switch to ATI. And no, and nVidia card can be used on a crossfire board and an ATI card can be used on an SLI board
With stuff there it's just about $300 and well, it'l be better than what happaflap posted because for the CPU and RAM that I mentioned. O yea, and the CPU, you can also go with an X2 really, each one afaik comes wiith a nice combo of either RAM, CPU, PSU etc... I forgot what kit it is, but there's some 2gb ddr2 800 kit that combos with a 1 or either 2gb flash drive which adds liek $5-10 so there's something too. I'll link to teh stuff later if you want but yea, it's all on newegg