This is what I'd do - 5 places to check to start you off...
1. Have a look in your local paper, free ads, etc for similar PCs
2. Have a look on eBay for similar PCs & their prices
3. Visit sites like eBuyer, Scan, etc. & see how much new similar/equivalent components cost. Then, your PC being secondhand, knock at least 25% off
4. Dells are usually very good value. Look on Dell for a similar PC - an example of one you could adjust to your spec click here - then knock a bit off
5. Secondhand/refurb PCs/Dells are even better value: click here & click here
In your evaluation/comparison, don't forget to also take into consideration the case, power supply (PSU), optical drive(s), dedicated soundcard, OS installed, etc.
Quality of components could be a factor too. E.g. your RAM is better than what you'd find in an average Dell. But how many people can tell the difference / be bothered / be willing to pay more? G