Even if you got the A64 to 2.6ghz it may not be better then the 2.6ghz of the FX55...Why??? cause the FX55 has 1mb of l2 cache.....NOw if you figure in the 289 or so HTT from a 3000+ and what ram divides you are able tio use it could give you more bandwidth...
however as mentioned the FX55 will have headroom as well and should clock to 2.8ghz no sweat....3.0ghz is in site but gets close to limits of air cooling alone.....
At 2.66ghz I beat the FX55 in every app out there that isn't effected by vid card or is extremely IO dependent....AMD thinks the 512kb mor of l2 cache is worth 5% hence why they give the 4000+ a higher rating over the same 2.4ghz speed of the 3800+...howvere they are wrong and this is not across the board...My 66mhz, higher bandwidth, higher HTT, seem to make up for it and then some....