id say if grab a
EVGA GeForce GTX 460 Superclocked 1024MB GDDR5 £150 approx
sell your EVGA GTS 250 for £50 id say you'd get
- basically as you suggested but if you do it with that model you spend all of £100 and it will mince the 768 edition GTX460's as well as most 1024 versions. -- so you can figure the potential with SLI of this edition..
Now the core2 quads i believe your Q6600 is im unsure on chipsets, as their are 2 LGA775's but im unsure the difference of the G or P series..
price to power ratio (due to your overclocked Q6600 you're probably better off not paying out for an I7 yet. but dependant on your LGA775 chipset limiations you could be stuck with DDR2 system ram)
But to answer your question a price for that as if you were to sell as a whole id say
Asus p5Q - £50
Nvidia GTS250 - £50
Ram 8gb - £40-65 (if your lucky buyer might be stuck with DDR2 mobo and have no option but to pay the discounted premium this ram used to cost)
Antec 900 - £40
HDD - £25
3 year old PSU i dont think you really can put a price on and im unfamiliar with coolermasters manufacturer warranty. i believe it is 3years.
Soundcard £15 tops id say
Windows 7 obv you wont be selling exactly
CPU cooler = cheap as chips probably wont fetch any cash on
Intel Q6600 OC'd - if u can sell it stable at 3.2 id say you might even get £100 for that puppy.
so on a good sale id say you're looking at around £300 plus as a system.
(prices based on 1/2 retail price of components taking component age into consideration)
hope this helps