Well, brand new your mac was £899 + £820 + £140 = £1859 (thats IF you bought the RAM from Apple). I dont think you will get your investment back if you did buy your RAM from Apple, as it can be bought L square for about 1/2 the price. Applecare is also good, so im going for rougly:
£1200-£1400, depending on condition. You could have gotten more before the Intel iMac I think, but now they are out, I think you will have incured a drop in value.
Out of interest, why are you selling it? Going for an Intel or what?