If you buy one of thier ready made systems off the shelf you'll have no problem with it.
If you later want to upgrade your computer I wouldn't bother asking them, the knowledge of staff there seems to vary wildly. At least the ready made systems are built to a good standard.
In short, buy your comp from there but in 2-3 years when you want more memory, better gfx card etc, get advice somewhere else and then buy the parts from Maplins and then fit the bits yourself.