Normally you would benchmark a system against some of the most demanding programs like CS4 or Crysis.

An extreme gaming machine priced around $4000 will give you 100% at full settings, while a good dollar per performance machine will give you 85% for $2000.

You can see the price difference for the small amount of gain is not really worth it if you’re fine with the settings as they are.

 You will also need a screen bigger than 24" to connect your pc to in order to make a top video card slug with framerate at all.

Video - ATI are on top currently as best price for performance and perform anti aliasing better, NVIDIA’s top end GTX280 kick ass but are expensive ($450+) each, for that you can buy a couple of ATI's 4850 range and run them on crossfire.

 Ram - DDR3 IMO has come a long way this year and proven the latencies (transfer timings per signal to/fm the processor) are achieving better results vs. DDR2 with half its frequency.

 Hard drive - matters not unless you want a raid setup to access files faster (e.g. server, video editing/rendering), do you really need 1000 GB or that super flash veloceraptor?

 Processor - quad core will get you further through games/programs/apps written with those set of instructions in mind, crunching raw data like archiving and rendering. A faster core clock of a core 2 duo (Intel in mind) will do you fine unless your wallet has no bottom in that case splash out on the new Intel i7 architecture with a $430+ motherboard and triple channel ram.

 Cooling - is dependant on your environment, what component setup will generate heat and the all contributing factor involving environmental damage and energy usage. passive = a heat sink, usually tin/zinc and copper for performing ones, fan driven = uses a fan on top of the later, fans with more blades and a larger span will produce more CFM for less the noise, liquid cooling will give you the best publicly available solution (save phase change) if correctly installed e.g. not having flow direction bypass the GPU from the CPU to the reservoir. One or two fans is sufficient = an intake and exhaust.

 Psu - it’s important to run your investment on a proper power supply. One that delivers regulated current on dedicated rails, EMR/RFI shielding, circuit switching, power down capacitors, and a good warranty will be the difference to name a few if you buy one valued around $150 vs. one with the same rated wattage drain supplement at $50.

 At most you can recycle your case, burner and hard drive. Then you need only buy the essentials like motherboard, RAM, video and a decent PSU to run it.

 It’s all down to what you want it to do, most gaming machines/high lvl graphics editors are good for 18-24months, office machines 24-36 months, and something that plays pong and can type about 10 years like the one you already own....