At that resolution, the 8800GTS will be the best bang for the buck.
From what I can remember the 8800GTS compares very well with the 8800GTX. The GTX only out paces the GTS at higher resolution, and I don't believe it was by too much.
Not to mention the (G92) GTS is manufactured using a 65nm process instead of a 90nm process and should use less power/ generate less heat. I wouldn't got for a GTX for an extra $40.
As stated, both cards are nearly equal in performance. They trade blows in different games - 8800 GTS 512 gets the with the edge at the lower resolutions, and the GTX is better a higher resolutions/high IQ levels. As you can see in the game averages below, both cards are nearly identical in performance with a tiny advantage to the GTX.
If I were to find a 8800GTX at a comparable price, though?
8800GTX for £230 at www.overclockers.co.uk
Not much more than an 8800GTS G92
Although, for £230 a MSI NX8800GTS 512MB can be had, with 730mhz Core & 1944mhz DDR3... which will definitely give the GTX a run for its money whilst remaining cooler & using less power.
"Our today’s tests show that this new graphics card from the $299-349 price category is indeed as fast as the Nvidia GeForce 8800 GTX, which comes from the $549-649 sector, in most of games despite having less memory and lower memory bandwidth. The only exception is the resolution of 1920x1200 where the influence of the memory subsystem parameters on performance is substantial especially with enabled FSAA"
A 24" monitor runs @ 1920x1200... well mine does anyhow