Well, that is sort of a difficult decision. Personally I wouldn't bother with 24-inch setups. Ever since I got my 28-inch I don't even use my 42-inch TV for watching TV/movies. So, there are more benefits a large monitor gives you than just gaming related stuff. Heck, few weeks ago TigerDirect had a 28-inch screen on sale for around $255-$260 with free shipping. That is pretty close to what decent 24-inchers cost nowadays.
It all depends if your games are running fine with your current monitor/res. I mean, if you are having problems with some games at 1440x900 with you current video card, then you are definitively going to have problems at 1920x1200 with that same card in those games. If you can max out your games at 1440x900 then get a new monitor, but you might need a new card since 1920x1200 is going to put a strain on that 4870 of yours.
Honestly, I would get both and be done with it. Get a monitor first and then get a nice card a month or two down the road once prices drop due to Fermi release.
Note: By the way I have a 19-inch Hanns-G monitor serving as my auxiliary screen. It is a pretty decent monitor.