Basic user-upgrading of that machine is a lost cause if you want "a high end gaming machine". You can put all the extra RAM in you like, and get a nice graphics card, but you're still going to have that horrible Celeron chip panting away in the middle of it all.
A PC is only as fast as it's slowest component. A quick search shows that you can buy a dual core Celeron clocked at 2Ghz (faster than yours) for �35. So spending loads of dosh on extra components for that processor to bottleneck is pointless. But, to change the processor to something decent, you're looking at a new motherboard and a job that's suited to someone who knows what they're doing.
If you just bought this machine new, take it back and say you want a refund. Even if it's a bloke in a shop, you have your statutory rights. You wanted a high end machine, and you got sold a very low end one.