I bought an oem advent machine a while back, which has lasted me pretty well. The model number was t9509, spec list of that machine is here http://www.uktsupport.co.uk/advent/pc/t9509.htm.
I mostly still have the same components as that list, except for an upgraded psu, to 500w I think, and a radeon 6850. Other than that, same parts.
Currently it does not run the games I want it to very well, I suspect it may be partly due to the 2gb of memory it has, but also that the 2.4ghz quad processor is a little slow. RAM I can get, but I understand the processor (q6600) is quite overclockable, and I was keen to give this a try. Mobo is locked though, so I think I need a new one. This hits the limits of my computer knowledge, not sure what would be a good upgrade, or what I need to look for. Would something like