the opteron 180 is more expensive. you really need to hurt whoever said the 4800+ outperforms the FX-60. the cores are identical except the FX-60 runs 200mhz faster!!!
maybe in particular task yes. the 4800+ will spank them at multi-tasking and encoding. clearly the FX-60 would outperform the opteron but i would still buy the opteron, its a heck of a lot cheaper and will easily overclock to 2.6ghz
DDR2 is not compatable with AMD. it is pretty poor as a memory platform. it does allow for higher frequencies than standard DDR but at the cost of tight latencies. there is no limit to overclocking. it depends on skill, components and cooling