New Computer.....self built? Mac? Dell?

[Mondo]

First of all, I haven't bought a new PC in about 8 years and not upgraded the CPU for 4/5 years. Well apart from getting a new GFX (ATI X800 XTPE 4 years ago) and upgraded to a Widescreen TFT around the same time, with a new PSU and more ram. It is still the same as P4 3.2Ghz, 2G DDR RAM (not even DDR2) as before.

It is time to get a new rig now, I am open to anything.

Options:-

1 - Get a iMac 20" or 24". Reason being that these days i don't use CAD (i did when i had the last machine), and these days its all Web, Photoshop and Lightroom. So I thought i would give Mac a chance.

2 - Get something like Dell, just base unit as i already have 2 monitors i can use it with...which also means they are a waste if i get an imac and i think a mac mini is not enough.

3 - Self Built PC....could do but i have no idea how to install the mobo and vaguely knows how to install the CPU and heat sink.

Obviously the imac is the most expensive but i think the price can be justified. A Dell unit often can be a bargain as it comes with an OS pre-installed and legit with warranty on the entire unit. Self built is cheapest and i can use windows 7 for the next 7 months at least.

I want something with at least 4G of ram, Quad Core if possible, low spec GFX would be fine, just enough to run HD res. HD wise, i am just going to get external from now on i think, with 1TB drives like £60, its easy just to fill up, and then back up and put it in the cupboard or something.

I guess the question is, i don't know what spec is good, what can one get for £600 PC wise? I know why imacs are as they spec are fixed on the apple site. But with PC i am very lost, which DDR are good?

Can someone spec me something for say £500 to £600? including case, and PSU.

P.S. I wanna go Intel, not AMD.....bad experience....