Selling current card, best upgrade option?

Hey everyone. I'm pretty new here to the TH forums, but definitely not new to computer hardware.

Here's what I currently have:
Q6600 B3 (usually stock 2.4GHz, though sometimes bring it up to 3.2GHz. Temperatures suck thanks to the stepping, so it's usually at stock)
4GB DDR2 800MHz
Gigabyte GA-P35-DS3R
3x500GB drives in RAID 0
Razer Barracuda AC-1 sound card
Visiontek 4870 512MB
24" 1920x1200 monitor
Windows 7

My friend offered to buy my 4870 for $150 (not too far from what one could find for the card new on Slickdeals) and I was trying to figure out what to upgrade to. The 4870 X2 would be amazing for bragging rights, that's for sure, but would it really be worth the extra money? I can snag a 1GB 4870 for $185 ($35 after selling my card), which would help with games at my monitor's native resolution and would allow for higher settings in GTA IV (definitely a plus), but a 4870 X2 seems substantially more expensive than that.

Is it worth the money to get the X2? Will there be that noticeable a performance difference between the 1GB 4870? From what I've come to understand, the drivers are still far from perfect for the X2 version.

How about one of the offerings from NVIDIA? I'm open to switching back to the green side if the performance/price ratio (remember, take $150 off any card to account for selling my 512MB 4870) would make an upgrade worthwhile.

Any opinions on this would be great.