I have a HD 5670 (PCI-e x 16 2.1) in a (X 16 1.0A) and it works well.
have to be careful since some 2.1 cards have problems in 1.0 slots
make sure you update your BIOS before installing the card.
Mobo makers sometimes had to do an update for the newer cards to work.
Any HD 5xxx is really a 2.1 even if the ad states 2.0
but the HD 5670 worked fine for me.
the HD 6670 cost about $30 more than the HD 5670 for only about %10-%20
really only about max 2-3 FPS difference in games
so that is something to consider if budget is a priority
the HD 6670 will have newer features for example UVD and ATI Stream etc
so if the price difference doesnt bother you than go for the HD 6670
BTW the 1gb wont make a difference with either of those cards
the HD 5/6670 series is not fast enough to take advantage of the 1gb Vram
I went for a 512mb HD 5670 GDDR5 card myself
just to let you know I play the following games if it matters
Crysis Warhead 1920x1080 DX10 Gamer,1680x1050 DX10 Enthusiast
Crysis 2 1920x1080 DX9 Hardcore no AA
Metro 2033 1920x1080 DX11 high settings no AA
Witcher 2 1920x1080 high settings no AA
that is with a C2D E4500 2.2 OCd to 2.93 4gb ram
so you can play higher settings at 1920x1080
with no AA for most games
definitely max settings at 1680x1050 for most games
My system is almost 5 yrs old and have not upgraded it ever since. Had bought all the part piece by piece and had assembled it myself. Thereafter I've not been in touch with the latest techno stuff and hence, am planning to align myself for the next year's upgrade..
Core2 Duo 1.8Ghz (had bought it on the day of its release - I may try overclocking it but will need expert guidance)
3 GB DDR2 633Hz
Intel D945GZIS Motherboard
450W power supply
17" CRT monitor
500GB + 320GB HDD
Gamepad - similar to PS2
Motion Sensor pistol for shooter games
Budget is the best and lowest fit to run games like Mafia2, COJ3 or COD latest.. dont want to spend on upgrade since I'll part off with this system next year as am sensing few mind freaking updates from internal sources on Intel. Hence, waiting for the new developments.
Next year wont mind to shell out $1500 odd to have a new machine and hence, am limiting myself for now.