FYI I have both... actually had the 4150... returned it. Have the 2800T now.
2800T has 15" SXGA+, Radeon Mobility 64MB 9000, P4 1.8M, 512MB PC2100, 5400 RPM HD, 24X CDRW/DVD, free (wifi, case, XP Pro upgrade) and $100 off coupon. It's over a pound lighter and the battery life is better than the 4150. All for less than $2500.
The reliability reviews don't mention the wacked out keyboard on the 4150 that slopes down towards the monitor hinge which leaves the front of the keyboard positions high and rather uncomfortable. It's also important to look at who is sponsoring those surveys... very few surveys are without bias.
My own personal experience with laptops is that Dell has been the most unreliable and finnicky of the bunch. I do have to commend them though for regular BIOS and driver updates, something the other laptop manufacturers could take a lesson in.
4150 also had a mono speaker... 2800T has the JBL Pro's... IMO best sounding out there for a laptop, but still laptop speakers... meaning don't expect terrific base or anything of the like...
If you do, open laptop, place head on keyboard, slam laptop closed on head.