if its second hand without any upgrades you may be lucky to get £150 for it.
core 2 duo is thier budget cpu's the hdd isnt overly high in capacity, the rams fairly decent but still minimal and the gpu isnt up to much.
then add in the fact its second hand, vista, and only the tower, really wont get to much for it.
you may find that stripping it down and selling it off as parts may be your best bet.
say £170 for the mobo, cpu and ram as a bundle, can try listing it on ebay for £200 as a bundle, then £20 for the graphics card, £15 for the case and psu together and £10 for the dvd drive.
if they all sold that would net you a total of £315 but the dvd drive probably wouldnt sell nor the GPU.
try that on ebay. also buy some anti static bags for posting them in then pack them in buble wrap and jiffy bag, or a box for the bundle instead of a jiffy bag.
if you sell on ebay, use first class recorded delivery for ALL listings and paypal only and dont accept local pick up if they pay via paypal as anything other than this can cause all kinds of problems like chargebacks, item not received claims and so forth.
oh and as for the hdd. you can sell this off as a formatted hdd for £30. if you do sell it off in parts, remember to remove the vista COA sticker from the case. keep it somewhere safe as then you have your liscence still.
you CAN put up the price of the case if you leave the COA sticker on and supply installation disks for vista as if you DONT remove it and you sell the case you are effectively selling your vista liscence and will no longer have any rights over that product key.
hopefully that will give you some informed info, try to give as much info as i can do anymore questions pop them up.