Yes, they are more or less the same exact card, use the same G92 chip, & have the same amount or Stream Processors, about the only difference is the clock speeds of the Core, Shaders & Memory, resulting in slightly more bandwidth & Texture fill rates...
8800GTS 512MB:
Number of Stream Processors: 128
Core Clock: 650 MHz
Shaders Clock: 1625 MHz
Memory Clock: 1000 MHz
Memory Interface: 256-bit
Memory Bandwidth: 64 GB/sec
Texture Fillrate: 41.6 Billion/sec
9800GTX 512MB:
Number of Stream Processors: 128
Core Clock: 675 MHz
Shaders Clock: 1688 MHz
Memory Clock: 1100 MHz
Memory Interface: 256-bit
Memory Bandwidth: 70.4 GB/sec
Texture Fillrate: 43.2 Billion/sec
Other than the extra SLI connector on it & slightly different clock speeds, their for all intents & purposes the same exact card, just marketed under 2 different models...
also thats the only reason I suggested that one 9800GTX on Tiger, since it was pretty much in the price range of the "older" 8800GTS 512MB cards, otherwise, one of them could be found for that $270 on down to as low as $200 (have seen a couple of clearance/refurb/openbox deals around & one or two sites for them as low as that, but they didn't last long LOL)
Oh & forgot, I think, can't remember for sure, but I think the 9800GTX uses better Memory chips, if memory serves me right I seem to remember reading that the 8800GTS G92 models were already pretty much peaked out at their best clock speeds, many people were lucky if they could get another 10-20MHz out of them (The 8800GT's were the good Overclockers when the new G92's came out)... but the 9800GTX when released was supposed to allow for higher clocks due to different memory used from the 8800GTS G92's...