Biggest thing you could benefit from at the price you are looking to pay is the high clockspeed of the current midrange CPU's. That's where I see the biggest improvement as compared to your current setup. Your chip now is a duo-core, but faster processing clockspeed -> multithreading doesn't lock threads as long.
The specs you have are really nice otherwise. So I'd honestly recommend buying a nice chip for that $500 you are looking to spend. Unless of course, you intend to spread those 8 clients out on to two cpus. Because that'd be faster yet, spreading both network traffic and CPU processes out.
Two questions for you: What model motherboard do you have? (you can find this through command prompt) and what is the current type of memory that you have in your system (DDR3, DDR2, etc..)?
I've included Tidus's link below from the shoutbox. Those are solid PC's, but I don't think you'll be able to beat the 8GB of RAM that you have, unless you currently use something less than DDR3 RAM. There are significant speed improvements between the memory types.
http://www.pricewatch.com/price/computer_systems_with_os/i7-2700k