£40 was quite a reasonable offer since the 1tb drives are selling for about £50-60 new and a caddy is basically free.
The HP Slimline
Its running an E2140 1.6GHz Pentium Dual Core
With that form factor it will be running on a micro atx board so upgrades will be rather difficult. It'll be running on board graphics so gaming will be a no no as would HD playback.
Its running 32 bit Vista, prob Home basic. With it been 32 bit it will not read more than 3GB ram total, that includes the ram associated with the on board graphics.
With the small form factor of the board i would hope you have two DIMM slots so you could pop more ram in.
Vista runs ok with 2GB so it'll be sluggish with the 1GB installed.
This PC's target is web browsing and office word processing.
As for a value for it you could squeeze £120 delivered. 2GHz dual core with 2GB ram are going for £150