Dell are ALWAYS over-priced for the spec. Avoid.
Beyond that, it's pretty easy to compare and tell what's a good price and what isn't.
I have an HP desktop that I have no complaints about, and an Asus touchscreen laptop which is pretty decent.
Would avoid Toshiba, and Sony are over-priced.
8/5 Blaze Bayley
18/5 Flotsam And Jetsam
25/5 Gov't Mule
29/5 Dylan Moran