Upgrading htpc gpu for gaming

About a year ago I built an htpc simply for movies, music and light gaming. I mean light gaming in the sense that I was playing old retro games like NES and PS1...lol. I love those old games.
Then-----Fallout 3 comes out. crap. So there I was playing fallout 3 on a 27" crt tv . I figured I had no need whatsoever to upgrade my video card in order to play games @ higher resolutions if my TV wouldnt display those resolutions. So, I played through fallout 3 at low settings and several other decent games settling for the less than stellar graphics settings. A week ago, my wife gave me the go ahead and we purchased a 47" 1080p LCD tv. Now, I want to play those games again at higher res so I can finally enjoy the games the way they were meant to be seen. But, in order to do so, I need a GPU upgrade.

The htpc I built consits of:

Asus M3A78EM AM2+/AM2 780g based MOBO

AMD Athlon 64 X2 5000 brisbane 2.6Ghz socket AM2 CPU

OCZ Platinum Revision 2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)

Hard drive isnt that great, but its a 7200 sata 3.03.0gb/s, 500GB drive

I have an ATI HD3450 runiing in hybrid xfire with the onboard HD3200 gpu

and, here is the kicker, im running XP pro

This setup has been more than adequate for movies of all types, bluray, hd-dvd, upscaled DVDs etc. But needs a boost for gaming. I would like to play games such as fallout 3 @ AT LEAST 720p resolution with fairly detailed settings. Playing at my TV's native 1080p res would be nice, but probably out of my budget.

Given my current system, I would like to get the best gpu i can afford but do not wish to upgrade my cpu at this time. So, i need the best card I can afford but do not need to spend the extra money on a card that will be bottle-necked by the CPU.

What would be the recommended card with my current setup in the sub $100 range that would give me the best performance/bang for the buck? I would think that my best bet would be the HD4670 since it would not require external power from the PS, but would something like a 3870 or 4550 (or others) be adequate for 720p gaming at high to very high settings?

I have no need to play games at higher that 1080p res seeing as how my tv would not display the extra res anyway. The extra headroom would be nice but....

Sorry for the long post. Thanks in advance!

Ok, so 1080 is out of the question, figured as much. What about 1280x720 or similar?

An HD4670 will max out most source games (L4D@Very high, etc) at 1280x1024 and similar res's.

The HD4670 is $60, and is the cheapest performance card you can get.

Just trust me on this. If you're thinking of saving, buy a phenom quad-core and an HD5770. That'll cost you ~$300 (I duno prices of Phenom q-cores...), but it'll be the best you can do...

Actually, you can buy a Phenom II x2 550, a $60 mobo, and an HD5770 for that price..

I would love to do that, but cannot. I just built this PC a year ago and saved a considerable amount of money by waiting, finding the best deals and using rebates. If money wasnt the issue, I would get the 5770 and max oput my cpu & memory. When I mentioned saving, I meant maybe saving up another $20-$40, not $240!

And a new power supply, so basically a new card will cost me the cost of the card+new PSU. Looks like the 4670 would be my only option if not upgrading my PSU