I would personally go with either of the Intel CPUs. As for the choice between the Xeon 3000 series vs. the Xeon 5100 series, you probably will not notice a significant difference between them. However, I think the 5100 series can be used in pairs (2 physical CPUs on a compatible motherboard) while the 3000 series cannot. The 5100's also support FB-DIMM RAM (the 3000's use DDR2) and many 5100 Xeons have a 1333MHz front side bus (the 3000's have a 1066MHz FSB).
I'm sure many others on this forum can elucidate the differences far better than I can.
Ive had a lot of experience running cs servers, If you dont allready know CS or CSS dosnt multithread and thus only uses one core. so your looking for the highest ghz for your money. also ask them what memmory they put on there boards as bandwith is important. hard-drive space is pretty irrelivent but a fast drive will help on map changes. I hope this helps. forget the opteron I had one and there not that fast.
would prob give you a better gamming experience, over the other low end xeons. but its not all about the processor its memmory and motherboard. you could try connecting to my server to see what smooth feels like but its empty at the momment prob at this time due to people only just waking up over here lol.
also check around on the net, key in the processor information and get reviews on the processors, benchmarks, getting a games server at its best is a challange.