PCIE is backwards compatible gents. So having a PCIE 1.0 board won't make a difference.
To answer your question, at $250, I'd go for either tha GTX 660, or the 7850. It is said that the GTX 660 outperforms the HD 7870. So unless the price on the 7870 drops some more, I'd say go with the GTX 660. http://videocardz.com/34684/geforce-gtx-660-is-faster-than-radeon-hd-7870-costs-249
Some benchmarks for ya.