Well im sure you will get some good advice here.
Though to be honest I find alot of the latest games pants and I nearly always play old games that dont need hardly any of the power from my setup. (Which is 2 years old now...maybe more)
Many of the games are bad ports and no matter how powerful your computer is, it runs badly, not that I have had many issues, but many people do!
My PC, currently i7 950, 6GB RAM, Geforce 470, Windows 7 64bit, runs nearly everything on Max. Actually only recent game is struggles with was Metro 2033, but that looked really bad anyway and was a very poor example of a game! (My opinion). I have maxed Crysis out and also increased draw distance, so no pop ins and much more vegetation in the distance.
As I do lots of rendering and 3D stuff I find the extra power I dont need for games really handy. However, that depends on what you want to do. Games will always want to eat everything up, but you can likely get away with alot less then an i7 CPU.
I guess my advice would be not to worry too much about CPU power, but get something with Quad core. Get at least 6GB Ram, 8GB is ideal. However, most importantly get the best GPU you can afford, but also try to get one with the most VRAM you can afford. Finally, get Windows 7 64 bit. You need 64bit for memory addressing and everything I have runs on it, even the really really old stuff. Sometimes you have to do specific things to make something work, but its pretty easy to do. Oh and if you buy a new screen, research alot into one and then do some more. At the end of the day you will be looking at the screen alot! I nice 24" is the min size I would go for and as for make I love Samsung!
Anyway, thats my offerings :)