The PC you linked to looks to be good, however if you are quite serious about gaming and want it to last (i.e. not to have to get a new one in a year or two) I would consider upping the graphics card on it (which at the mo is 256, pretty much the minimum standard for gaming now), and possibly the RAM to 4GB, since today's games use lots of it. Add to that the fact that will be running Vista, which takes a minimum of 1GB RAM to run on its own... you get the picture

A good gaming machine that will last a few years will probably cost you just under a grand. Also, I'm not sure on buying a PC from Tesco. After sales service, support etc. will probably be crap.
You say you're not bothered about monitor and speakers, but monitor is very important, maybe speakers not so much. For gaming you'll want a 5ms response time at the least, you probably want it to be at least 19". Dell make fantastic cheap monitors, belive it or not.
As for Mac vs. PC, stick to a PC for gaming, as only the super popular games are ported to the Mac platform.