There are PCI-e v2.0 and v2.1 versions of the HD 5670. Since I have read of some people having issues with a PCI-e v2.1 card in a v1.0 slot, it's best to select the v2.0 version of the HD 5670.
The following link is for a Sapphire HD 5670 512MB PCI-e v2.0 card for $80 + $5 shipping. There's also a $15 mail-in rebate to bring the price down more. There is hardly any difference between a HD 5670 with 512MB or 1GB unless you care about a 1% or 2% difference in benchmarks.