Umm... Yes, 1.0s are backward compatible, however its just slower. between them all. Get the 4850s they are the cheapest and best preformance. the 4770s on newegg are 110 at lowest but preform worse than the 4850s. the 4850s are only 99.99 on newegg but are faster and better. However the 4670s, probably if not worst better than the 4770. It's going to have the same bit-rate and DX11 has a long way to go before it goes a mainstream thing. So 4850s are your cheapest and best bet.
The 4870s are good to though if you wanna pay the extra for it.
NVM just checked again to. With that PSU this http://www.newegg.com/Product/Product.aspx?Item=N82E16814131162&cm_re=hd_4850-_-14-131-162-_-Product
is the best bet for you. The 4870s. the cheap ones at least are going to push your budget cause the cheap ones are all 2x6 pins and the PSU your eyeing only has one.