Actually, the main reasons some people run sbox off dedicated servers or vps' it two-fold - to avoid tying up their local workstation *at all* (e.g. when in the slow poster mode, the app has to have focus and will take it away from any other app), and to help mitigate ban hammers on their local IP addr.
Same is probably true for senuke as far as freeing up their local machine and IP. Neither program is extremely memory- or resource-intensive. Biggest bottleneck with either is likely to be your ISP bandwidth if you're really pushing them.
A gaming machine would be a waste as a lot of the $ in them is on high-end graphics cards, which aren't really needed for either the apps you cite. A machine with a boatload of RAM and a low-end gfx card would beat a "gaming machine" for what you describe.
As for the AV app you're running, it's probably like a lot of AV apps on the market these days - buggy, resource-intensive, and slowing everything down (not just the two apps you mention). There's plenty of debate in many corners over which apps are best, but the one you mention doesn't come up often.
Best bets IMHO are:
use malware bytes and ccleaner regularly on any system you are bh'ing on
for freeware: Avast + ZoneAlarm is not bad
for paid solution: Kaspersky Internet Security has a kickass price on a 3-seat license.
If I were you, I'd start with the relatively small investment in replacement security software on your current workstation, then see how your results are.
