In reply to wilkie14c: I'd plump for more watts on the power supply. Maybe around 600+.
Go for a decent case with plenty of cooling options. Do you have space restrictions, mid or full tower?
I5 2500 is a very good processor, silly value for money last time I checked, high end and very overclockable. Might be something thats taken the highend bang for buck title since. Check the case dimensions for a cpu fan, most gamers replace the stock one for a much larger one. Nothing you have to buy now though.
Ask him what sort of games he wants to play. 1gb graphics card may not cut it, there are a lot of different cards at 1gb though, some older flagship cards and some new ones that arent nearly as quick. Quantity of ram isn't the most important statistic on a gpu. expect this to be the dearest component.
Get at least 8 gig ram to start with, ram is cheap. 2x2 is a bit old hat now. 8 is plenty and allows a considerable upgrade to 16 without binning 2 sticks of 2gig.
Get a good motherboard, check compatibility with other components. Most scrimped component usually but you can look to future proof a bit with a better one.