Can I run a PCI Express 2.0 card in a motherboard that doesn't support 2.0? If so, is the card performance bottlenecked? How badly? Not sure of the motherboard but the system is a Core 2 Duo 6600 w/4 GB RAM (667 MHZ). Current Video card is a Sapphire Radeon X1950 Pro.
What's the best Express v1.1 card out there for less than $100? Or, would a 2.0 card be better?
I'd personally get a 2.0/2.1 card in your situation. If you upgrade in the near future you'd be able to utilize the extra speed then.
In which case, I'd get the ASUS Radeon HD 6670
$90 after MIR. So same price as the 1.0 card...
In general newer version of PCI-E have backward compatibility with older version. but for PCI-E 2.1 card they have a bit of problem with PCI-E ver 1.x. some the problem are solved with updating the mobo's BIOS but it depends on mobo manufacturer support. as for bottleneck PCI-E 1.x will not going to bottleneck any single gpu out there unless you going for multi gpu setup or dual gpu card such as GTX590 or HD6990.
AFAIK almost all PCI-E card available today built with PCI- 2.0 or 2.1 spec. so far only ATI 5k and 6k series using PCI-E 2.1. for nvidia i think they will skip PCI-E 2.1 and go straight for PCI-E 3.0 when more mobo makers implement PCI-E 3.0 to their mobo next year (if i'm not mistaken Asrock already have mobo eith PCI-E 3.0)