they are not bad. but for 340, you could sli two 9600gt's and get near gx2 performance.. and I am not sure(have not seen any benches, but beat out two 880gtx's?) and then you could sell your gtx and make money on the deal.
EDIT- I just saw what you posted, and if you want something on par with the gx2, then go for 9600gt's! and sell your gtx, either on here, or craigslist or something. Then you can afford to get a new cpu.
what mobo do you have?