I've had no problems upgrading Compaq, Dell and Packard Bell computers. Admittedly I didn't care about warranty, so I just bought whatever parts (disks, CPUs, RAM, CDs, cards etc.) that I needed and bunged them in.
No problems, no issues.
Of course, that was for home use, not work. People are much more pernickety in corporate environments
I'm using an Evesham PC, which is OK. My father and brother have both had a couple of Dell PCs and are quite staisfied. My brother-in-law has a Dell and had a problem that I helped to fix - I was favourably impressed with Dell's support website. Only Compaq's (HP) is better IMHO.
And don't forget to look at Dell's outlet store (good deals to be had).
Cheers mate
SD