Bit of an open forum question really.
Having spent an awful lot of money on my PC starting a year ago and ending just into this year, I have been more than happy and stopped looking at Tom's for fear of further upgradeitis!
So have been out of the hardware loop for quite a few months now (probably since April/May) and was interested to see how the new video cards are performing...
I was really hoping that 4870's/4870X2's would kick ass and it looks like they are and Nvidia are just throwing more and more transistors onto a card and cranking up the power usage; a fair assumption?
Anyhow, what's hot to have at the moment? Oh and good to see some of the old hands still around!
Maybe I'm remembered a little bit!

The time to upgrade is really when your current solution no longer makes you happy for whatever reason. Are there any games you need more performance in? Crysis DX10 very high is about all I can imagine.

A pair of 8800GTX's are still quite high end and more powerful than any single GPU made such as the GTX280 or HD4870. Your only upgrade options on that SLI motherboard, are the HD4870x2, SLI GTX260, or SLI GTX280. While they are more powerful than your cards, you don't have the CPU to take advantage of them. Even now, your CPU is a weaker link than your GPU's apart from maybe 25x16 resolution.

If it were me, I'd stay with your current rig and have your eyes set on an X58 mobo which will support SLI and crossfire. Then you will have numerous high end solution to choose from and can even just keep the 8800GTX's going from the start if you choose.

I'm still using a Radeon X1900XT 512MB in my gaming rig. It's holding up quite well for 1920 x 1200 eventhough it is a little over 2 years old. I'll probably wait for the next generation GPU whether it will be an ATI Radeon 5 series or a nVidia GTX 3 series (actual names my differ).

I did buy a passively cooled nVidia 9600GT for my HTPC. It's probably about 10% faster than my X1900XT based on some benchmarks.