that should be able to handle quite a bit, I only have 20 threads running using Dual X5660 with 32Gb Ram. I get around 1200-1300 caps per min with around 60-70% CPU so your shouldn't have an issue.
response time looks good, and the "missing" percent is failed captchas.
Few additional tweaks I am using is switch
Recognition:
Parallelism to Max
Disable Use adaptive system for setting parallelism degree
ReCaptchaSiteKey Addon:
Disable Monitor system resources
Disable Safe recognition mode (only if you have a lot of proxies, 10k+ for me)
Default Firefox Settings
Enable Ignore proxy from other software
Remove proxy from list if BANNED / TIMEOUT
Keep banned for (minimum) 24, just keep increasing by 12-24 hours if your banned/timeout keeps going up.
You also have more then enough resources for high number of RC2Instances and Max memory (currently for me its 400/400 default I think)
Oh almost forgot, dont forget to optimize your server as well.
TCPOptimizer - speedguide.net
VMWare Optimizer (great for physical servers as well)
Windows Servers optimization scripts
etc