Jack's actually wrong
, cards are NOT backwards compatible with base PCIe 1.0/1.0a , they are compatible with 1.0b (rare) and 1.1. This can easily be confirmed at PCI-SIG if you take the time.
There are very few graphics slots that only support the base in recent years since the announcement of 2.0 in 2007, but when the mobos are lane restricted they often do it like intel, MSI and ASUS boards, and then you end up with issues as described in ASUS' FAQ;http://support.asus.com/faq/asus-faq.aspx?SLanguage=en-us&no=DE05A01F-3A3B-A859-177B-6A668A2D6982
Most mobos that are listed as 1.0 in the past 2 years are likely 1.1 just generically listed as 1.0, but it's worth knowing for sure.
Double check the mobo mfr's site to be sure, or else make sure you have a good return policy on the card if you want to chance it.
The old announcement;
PCI-SIG announced the availability of the PCI Express Base 2.0 specification on 15 January 2007. PCIe 2.0 doubles the bus standard's bandwidth from 2.5 Gbit/s to 5 Gbit/s, meaning a x32 connector can transfer data at up to 16 GB/s in each direction. PCIe 2.0 is still compatible with PCIe 1.1, so older cards will still be able to work in machines with this new version.