I went from a 22" TN Acer to my 24" Soyo PVA LCD, and it was a very nice upgrade. I wouldn't say a "huge" upgrade, but games do look a lot nicer on my 24" screen, partly due to the bigger screen height ( alittle more immersive) and partly due to the PVA panel. I can say that once you use 1920 x 1200 on your desktop, yoiu may find it difficult to go back to 1680 X 1050. I'm not saying that 1680 x 1050 is not usable, it fine, but 1920 X 1200 is better.
The pixels are also a little smaller on a 24" LCD, and IMO they are the "ideal" size. If you go any smaller in the pixel size, you need very good eyes, while if you go any larger, you start to see the individual pixels, and things may look grainy at times. The 24" LCD isa good compromise in pixel size.