Personally, I've always thought macs were a wee bit overpriced for the hardware inside them, although a 24" does look nice. As for Mac OS X, I've (admittedly, illegally) installed it on this PC - I wasn't a great fan. Sure, you could do everything efficiently, the only problem was there really wasn't that much to do.
The Dell PC looks the business, although you'd want to make sure that the 256MB graphics card is separate from the motherboard, not one of those shared-memory jobbies that most computer manufacturers flog these days. The only problem (IMHO) is Vista - XP still runs a sight faster on all hardware - without the eye-candy of course.
The iMac is based around Core 2 duos from what I've seen, which are the fastest desktop based processor at the moment. However, AMDs Barcelona chip is supposedly up to 50% faster than a Core 2. I doubt if you'd be able to upgrade the Dell to a Barca-Core anyway.
Also, if you're doing any gaming (unlikely on a Mac, in reality), the Apple site quotes "NVIDIA 7300 GT graphics card, offering up to 60% faster performance over the 20-inch model.". This card is becoming outdated, with the current model being the 8800. AFAIK the 7300 is also the low end of the 7xxx series, with the 7900's being the high-end cards.
Building it yourself is a good option, although I dunno why you'd want a dual-19" layout - that's too wide IMO, although that's up to preference really.
PS. I do most of my casual browsing on Princes Street as well
I've not heard anything about an Apple store though.