I'm currently running on a Quad Core with 4GB of RAM (yes, I know 32bit applications can't make use of it, but I was developing 64bit apps at the time), and an 8800 GTS. It's adequate for what I do (it even doubles as a room heater), but really, I could use something with more speed, more cores and loads more RAM.
That you 'use only Radeon' is an interesting statement. It suggests that you diregard nVidia and other manufacturers without looking at their products.
I evaluated all the options before deciding to use the 8800GTS, including the Radeons, and concluded that 1) their driver support wasn't up to scratch 2) their GPGPU support wasn't up to scratch 3) they didn't have the same support in the gaming or CAD world (think Tesla, Quadro, etc). These have changed now, but that was the situation then.
If you think only a server needs 4GB of RAM, think again. Many applications these days will benefit from having huge amounts of RAM available - think CAD systems and geologic simulations. Think modern games (which given only a modest RAM boost can perform much better), think modern photo processing, think video processing (how is daddy going to process little johnnies football game on a celeron?).
To top it off, I've been using this system since April 2007 and I expect to be using it for another few years to come. The only thing I've changed is to add more RAM and storage. It's about buying quality gear upfront and having it survive much longer than cheaper equipment. I've got a Dell 600Mhz P3 still running under the desk as a file server (it's 9 years old this year).