Keep in mind that most MAME games use primarily the CPU, whereas most modern PC games use the graphics card primarily. So you need a decent graphics card to play most modern PC games.
Also, you may want to consider a USB xbox 360 controller (either wired or wireless). Many more games are becoming almost straight ports and a number are designed with that controller in mind (something I don't like much since I like the mouse/keyboard control setup, and games that are ported without the publishers tweaking the UI sometimes are screwy with mouse/keyboard). In fact, if I recall correctly, I belive that Games for Windows logo games must be designed so that an xbox 360 controller will work fine on them.
Again, this applies to modern games which you mentioned. Older games are differant beasts.