2 7950 512 MB GT's SLI vs 1 8800 GTS 640 MB

Hey,

I recently got a pc with the following specs:

Case - Tuniq Symmetry Gaming Tower Case w/500W NZXT PS
Processor - Intel Core 2 Duo Processor E6600 @ 2.4 Ghz
Processor Cooling - Stock HSF
Motherboard - Asus P5N-E SLI nForce 650i SLI Chipset
Memory - Corsair XMS2 Xtreme 1024MB DDR2-800 PC6400
Video Card - eVGA NVIDIA GeForce 7950GT 512MB
Hard Drives - 2 x [Serial ATA-150, 10,000 RPM] WD Raptor WD740GD 74.0GB 16M Cache Hard Drive (RAID0)
Sound Card - Creative Sound Blaster Audigy SE

Looking at my specs I figured I should upgrade my video card since I can get more out of this system.

What would be a better upgrade?

1) To get an 8800 GTS and basically dispose of (or sell my 7950 GT)

or

2) To get another 7950 GT and run both in SLI

Money-wise, an additional 8800 GTS (superclocked) would cost $410
http://www.newegg.com/Product/Prod [...] 6814130080

an additional 7950 GT would cost $229
http://www.newegg.com/Product/Prod [...] 6814130061

Although I haven't been able to find benchmarks comparing the two setups, would you think there is a noticeable performance diff between the two? Disregarding dx10 capabilities the 8800 has, which upgrade would I be better off with?

Some things to keep in consideration:

-Heat dissipation
-Power consumption (only a 500W PS)
-Performance
-Loudness
-Value for my money

Thanks for your input.

I agree with pete, specially if you're able to sell the 7950gt you currently have. You'll offset the cost and get an 8800gts for a little less!