Sorry but its a idea with loads of problems.
For a start. What parts of the Dell would you replace? If its the motherboard then you would need another copy of windows as it would fail validation.
Regarding upgrading Dells the problems are two fold mainly. Their power supplies are sometimes non standard which can cause problems if you need to replace them - or used to be non standard as I think this is no longer the case. The second problem is only with the cases really, some PCs are slimline cases and this cuts down on the items you can fit, you need low profile graphics cards etc. And this is not just a Dell problem its the same with any PC that uses a very small case.
Upgrading a processor might be impossible as the board might not accept it and even if it would accept the faster processor the difference in speed is not likely to be that great so it wouldn't be that good value.
Personally I would prefer a Novatech system.