I agree with
frostbite on this - upgrading Dell PCs can be a bit of a problem. You can usually replace Dell components with other brands, but adding additional can be a pain. I've had similar issues with power cables, drive mounting brackets and other bits - even IDE cables with a plug for only one device
The cheapest way to buy a Dell is via their "factory outlet" website - not necessarily old or refurbished stock, as they very often have cancelled orders (both home and corporate) available.
I haven't needed to look at it for a couple of years, but I'm sure it's still there. There were some amazing deals.
SD