well
the 7800GT is 20 pipelines, and 7 vertex shaders and i think stock clocks are 400Mhz core and 1000mhz memory. though alot of manufacturers are actually shipping cards with decent factory overclocks
the GTX has a further 4 pipes, bringing it to 24pipes, and an 8th vertex shader. the stock clocks for this are 430Mhz and 1200Mhz, again manufacturers are selling cards with decent overclocks
Nvidia have learnt though from the 6800 series. (ie where you could buy a GT and oc to an Ultra, or where you could buy a 6800U/ LE, unlock it in riva tuner and overclock it to ultra) so i dont think you can buy a GT in the hope of turning it into a GTX
as far as performance goes, obviously the GTX wins hands down.
but these two cards are CPU limited in a good chunk of games so actually perform the same.
if your wanting to play at Hi res and high AA and AF settings then plunking the extra down for the GTX will probably be worth it. if your stuck at say 12x10 (LCD) then maybes it might be a better option to save a little money and get a GT. then you can spend the savings toward a faster CPU or something. and if your not scared.....OC the GT ( 470odd Mhz is what seems to be the oc most are getting)