There's no right answer for the dual/quad debate. It all depends on your usage. If this is strictly a gaming computer then a dual core will be fine. If you'll be doing CPU intensive stuff that's optimized for quad core CPU's (video encoding, etc...) a quad core will be better. I personally do a lot of HD video editing and wish I had got a quad core every time I see my 3.2GHz dual core max out both cores.
I was trying to keep my options for SLI open going with NVIDIA and the motherboard I listed. I am asking anyone with a lot of experience, do you think that is advisable? Or is it just better to go with ATI's 4850 or 4870 cards?
The GA-EP45-UD3P motherboard supports crossfire which is ATI's version of SLI. The GTX 260 core 216 is SLIGHTLY faster than the 4870 1GB card. The Nvidia chipset motherboards are not nearly as stable as the Intel chipset boards. If it was me, I'd stick with the P45/4870 combo. That way you won't be losing much performance and will still have the ability to add another GPU later. Saving $116 along the way is just icing on the cake IMO.
shortstuff is right on the money with his suggestions. If you can find a better deal for a gtx260+ (216), then consider going for that instead. Be warned that SLI is not supported so if you are thinking of adding a 2nd card, go with the hd4870.