Is HD 1080p really worth it for LCD monitors?

If you are asking if it will help performance compared to the typical 1920 x 1200 resolution monitor, then yes it will improve performance because there are 10% less pixels for the HD 4890 to push. The fewer the pixels, the higher the performance.

If you are asking if it will improve your overall enjoyment verses a 1920 x 1200, then that is a different question that can only be answered by you. Will you miss the 120 rows of pixel?

There is no such thing as a "video card sweet spot". The higher the resolution, the more effort it takes the video card to push frame rates. Graphic quality settings and the game's graphics engine counts as well.

Crysis / Crysis Warhead and GTA IV are examples of poorly designed (GTA IV) or inefficient graphics engine. Those games will even choke the most powerful XFire or SLI setup at 1920 x 1200.

I am sure both monitors will work with Vista. Newegg (or any vendor) does not always list every single bit of spec data. They are even wrong sometimes. I prefer a 1920 x 1200 monitor; either Asus or LG will do at that price point.