For the "my monitor isn't big enough yet" crowd:
What better way to show off the new hotness in PC gaming than with a ridiculously large monitor? At 27.5", you're starting to tread dangerously near to HDTV ranges, but without the markup.
Hanns·G HG-281DPB - Link
For the "I still use the stock mouse that came with my Dell" people:
Come on, people! While any mouse is certainly just fine for most, a gaming mouse has features that will help customize your experience to one that better suits your comfort level. Things like DPI control and extra buttons (for my money, binding voice chat push-to-talk buttons to the mouse is the way to go) increase the level of control you have over your games, and that's never a bad thing.
Logitech G500 - Link
Video card upgrades
For the "solid but not bank breaking" crew:
I've got this card, and I haven't had any trouble running a game at maximum detail/resolution since I installed it. For the asking price, it's one of the best mid-range (price-wise) cards available, and c'mon! Who needs to bust their wallet to experience the best games the PC has to offer?
e-VGA nVidia GTX 260 - Link
For the "if it doesn't cost as much as a launch PS3, I don't want it because it isn't the fastest card ever" crowd:
...Oh right, that would be this guy. This is the fastest card currently on market, and this thing is actually more powerful than my entire PC was only a few years ago. Moore's Law strikes again!
XFX Radeon HD 5970 - Link