You can do some basic gaming with the HD4000 IGP. There are plenty of Ivy Bridge release reviews
out there to get a feel for what the IGP will do. At best, it'll be somewhat playable at 1366x768 with image quality (AA, AF, etc) dialed down. Haswell, next year's CPU, should double the fps.
When you move to a discrete card, here's the stuff to think about:
- what's the monitor resolution? (in your case, with an HTPC, the assumption is 1920x1080)
- what are the games? (Some are GPU intensive, some are CPU intensive)
- How much bling (AA, AF, etc) do you want turned on? This can make a staggering difference on $'s spent.
- How much do you want to pay?
- And since this is SPCR:
- how quiet do I want the card to be?
- how much power does it use?
Then go look at the latest reviews. They'll all show benchmarks of a variety of games vs a dozen or more different cards.
The short version for 1080p gaming:
low end ($200-240): HD7850, GTX 660
high end ($300): GTX 660 Ti
This generation, I like the Nvidia cards over AMD due to lower power use in 2D, BluRay, multi-monitor uses.