Games not running smoothly?

Hello!

So a few months ago, using an MSI P67A-G43, Intel Core i3 2100, asus radeon HD 6850 on stock, 4 gigs of ram, Antec Earth 650w PSU, and 64 bit Windows 7, I built myself a computer. I run it on 1920x1080 resolution (I realize this might be part of the problem.)

I researched the parts very carefully, looking at benchmarks and other statistics to be sure I was leaving myself plenty of room for upgrade (planning on running crossfire 6850s + an OC'd 2500k), and that it would run as expected on games like TF2. At first, it was somewhat smooth (but STILL not perfect - I don't expect perfection from a budget PC with a widescreen monitor, but believe me when I say something just seems 'off'); since then, the quality of play has deteriorated significantly. It plays more choppily, even on ultra low graphics settings.

It's not just TF2, or even just games. MINECRAFT seems to have some kind of weird stuttery thing going on (processor?). From the very BEGINNING it also appeared as though some Windows 7 effects, etc were choppy.

If there is no problem and it's simply an incapability of the hardware, fine. Show me the bottleneck, I can live with/replace it, but I want to make SURE that's the case first. I'm concerned I've already done something wrong, treated it poorly, or have in some way caused it to function less than optimally.

What do I do to figure out why things are happening the way they are? How can I help you to help me?

Thank you in advance!