Hmmm...sadly, $150 no longer gets you very far. I remember when $150 could get you a real performer. If I were you--and I know you're building a new rig--I would wait, save up a little more money, and go for an 8800GT. Yeah, it's in the mid- to high-$200s. But it'll be worth it. Another $100 will make you happy with your first $150, and it'll make you happy with all the other money you just spent on your rig.
Trust me, I cheaped out and got an 8600GTS (~$180). Performance in CoD4 is okay for singleplayer, I would be gimped in multi. But TF2 and HL2:E2 both like my card.
Actually, I max out all my settings (except AA) with the 8600, so maybe that would be a good place to start. Just make sure you buy from EVGA so you can always step-up without a hassle if you're not happy or find another $100 within 90 days.
Setting aside $1 a day will get you into something much nicer if you step up....