It's your call really. I'm in the same situation, and having had researched this a bit, I've decided that at this point, if you're sitting on a 8800GT/9800GT or greater, you'll definitely be fine for now until the next new cards come out. That's not to say there isn't some good better cards out there. There are, but honestly, despite the age of the GeForce 8800GT, stuff that's come out since then has been, well, pretty disappointing in my opinion. It's mostly been more renaming and small improvements rather than actual new stuff or big jumps since the G80 debuted. We used to get a new real series every year or so, and I thought the GeForce 6 to GeForce 7 change was small, but wow, these days it's so much worse. nVidia's spending more time renaming parts than it is making any.
If you do upgrade, the GeForce GTX260 and/or Radeon HD4870 should be the minimum you go for, but unless you go all out, you won't get the same "wow" factor you're used to even with those, and going all out on the current cards means alot of money. Of course the cards I'm waiting for won't be cheap at launch either, but I'd still rather wait for the gap to get bigger. The gap isn't quite at that point just yet. The GeForce 8800GT is still a very capable card.
Wait for some new technology, you may even need a new PSU as well when you put in this beast, if it doesn't have the required type of connections. The performance gain isn't worth the money I think. Especially not on a 19" screen.
Normally I only get a new card if the graphics settings start getting annoying to configure because I can't run the damn thing. At least now if things lag I can just turn the AA down one notch and I'm good to go.