What you also have to consider is that if you are running a 24" screen, then you would want a PC that can run the games at that screens native resolution which will be 1920X1200, thats fairly high. I would highly advise a 4870 with 1GB RAM on it to play games comfortably and not run into problems in the future...
Having said that, now is a horible time to buy a new pc, prices are through the roof on hardware, i7 is out and should get cheaper pretty fast... DDR3 is finally becoming more affordable as more and more system manufacturers are using it and I won even go into all the chipset and conectivity changes that come with the i7 platform. If I was you Id wait about 3 to 4 months before getting a new PC.
Oh, and have I mentioned Solid State HDD's? lol