For $90 after rebate, I prefer the faster HD 5770 which has been rebranded to the HD 6770. It does use more power than the HD 5670 / HD 6670 though. I recommend at least a good quality 450w PSU or an average quality 500w PSU for the card.
Older motherboards, like my Abit IP35Pro from mid 2008, only has a PCI-e 1.0 or 1.1 slot. They can have compatibility issues with a PCI-e 2.1 card.
If you do have a PCI-e 1.x slot, then you need to look for PCI-e 2.0 cards which are definitely backwards compatible. All Radeon HD 6xxx cards are PCI-e 2.1, and most remaining Radeon HD 5xxx cards are also PCI-e 2.1 as well.
Here are all the current Radeon HD cards available from Newegg that are PCI-e 2.0: