I'll say go for 6600GT because its cheaper and comes with SLI!
I'm not sure if the X800GT will support ATI multi GPU, but even if it did, you'll need a more expensive Master card and a whole new mobo.

Actually the performance between x800GT and 6600GT is similar in a number of games. If I let you game on rig with either one of these cards installed, you wouldn't probably notice the difference until you pop the hood.

Considering that the x800GT is a little bit more expensive and slightly better performance (in certian games only), I'll say it is not a good choice espcially when you look at the API support. More new games will need SM3.0 which helps to improve performance and image quality.

Nevertheless, you have to understand that the x800GT is based on "rejected" R480 GPUs. Thus overclocking totally depends on your luck and stocks are limited. Once the x800GT gets sold out, its gone..... no more.

Bottom line, get 6600GT; the best mid-range graphics card of the year! It gets better with the price of 6600GT dropping by the month.

P/S: I'm not being NVIDIA fan here but the 6600GT has more value for money and allows you to improve performance by dropping another card to enable SLI.

Thats surely a R480 under da hood. Run RivaTuner and you will see R480 there