I may confuse you more, but here it goes.
First off, what OS are you using to run the dedicated game server? Reason I ask is in my experience, and this is coming from certified Microsoft engineer, linux performs much better for dedicated servers often times, including for games. If Unreal has a linux package for dedicated servers, and you're running it on windows, your solution for better performance could cost you nothing aside from time to learn linux and how to setup unreal ds on it.
Also, your CPU utilization has not hit 100%, so why are you worried? Also, CPU utilization could be high because you don't have enough RAM, causing the CPU to call up the hard drive for frequent caching. Maybe you have too many services in the background you don't need. Have you exhausted these measures?
With that said, let's address the AMD 64 vs HT questions....
First, please remember AMD CPU's have for years now been able to perform better at the same clock speeds at Intel. This doesn't mean they're better, but more efficient per clock cycle.
Next, there's more to Athlon 64's than just 64-bit extensions. They perform exceptionally in 32-bit OS's because they actually have the memory controllers that handle memory calculations on die in the CPU, not on the motherboards, like Intel does. This gives AMD's a HUGE advantage in memory access over the Intel, even with Intel systems using faster DD2 RAM.
However, Intel does do better in other things. Hypterthreading gives Intel a marked advantage IF AND ONLY IF the application is optimized for HT, such as photoshop CS. However, I sincerely doubt your ds package is, I'm 99.9999% sure of that.
AMD CPU's for most of what servers do would probably work better, to be quite honest. And it's no surprise that while enterprise class servers have always been Intel dominated, even HP/Compaq and others are now building AMD based servers in addition to Intel based, and the AMD's are proving to be faster, and we're talking about much more demanding apps like SQL, Exchange, large web servers with tons of hits, etc.
But I REALLY think you don't need to replace your 2400 for an older unreal dedicated server.
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