However, I'm also in the process of updating my machine, so I'm trying to weigh the options of ditching the 8800GT entirely and upgrading to 4850s or 4870s in Crossfire and can't really tell if I'll be gaining THAT much in overall performance and future gaming.
If you are looking to the future then look towards the ATI 5XXX cards rather than upgrading with older tech.
However, the prices for those are still high and, while it flys in the face of my previous statement, I'd suggest you get the second 8800 for now as a useful, if short-term upgrade, then wait until Nvidia release their next gen cards. By the time Nvidia releases its new hardware the 5XXX should be a little cheaper-and easier to find-and there may be a viable (superior?) Nvidia alternative to install in the fully upgraded machine.
And I'm sure yopu could find a nice home for the 8800s' on E-bay;)
I'm not exactly sure how up to date the Tom's hardware graphics charts are these days; I've grown a little wary of the hiearchy, but I can say your best bet is to get another 8800GT. That will offer around stock GTX 260 192sp performance at 1920x1200. Please note the first 8800GT SLi in that chart uses two 1GB versions, which offers roughly overclocked GTX 260 216sp performance which is a little bit of a difference.
Regardless, that'll allow you the performance to last you until Nvidia releases their DirectX 11 parts, or ATi to lower their prices on their DirectX 11 parts.
I currently own two 8800 GT's and run them in SLI. I am able to run almost anything out there on maxed out settings with AA and anisotropic filtering using directx 10 when available. I have a 24" monitor and I game at 1920x1200. The only two games that I've come across that I have had to play on medium-high settings are the two Crysis games and Company of Heroes, but they still look stunning. I would say monetarily speaking, your best bet is to add another 8800 GT, which would offer you great performance at a reasonable price. Hope this helps.