If you go the Mac route, you will still need a reasonable amount of RAM, but you can get this so cheaply from Crucial / Amazon, etc that 12/16GB should be plenty affordable. Stating the obvious, the price difference between the 2 options does also include a 27" monitor with the iMac, which I guess would be £500-600 or so for something like a Dell 27" IPS monitor.
I personally moved to a Mac (27" iMac) about 3 years ago, and would not choose to change back, however I also use several flavours of Windows every day for work. Aperture vs. Lightroom is a personal thing - I tried both and choose Lightroom, but many people prefer Aperture. As others have said, you can trial LR4 for free so nothing to loose there, however I think Apple have now removed the ability to trial Aperture.
Downside of the iMac against PC is the disk, which is not straightforward to replace or add. You can buy them with an SSD fitted as well as a normal disk, but this bumps the cost A LOT and is very expensive for what they add, plus unless you want to go with external Thunderbolt disk (very pricey currently) the fastest external disk you can add is FW800, as they don't support USB3 yet. Apple are expected to be announcing upgrades to most of their machines next week, so you might be wise to wait until then to see what the changes are, if any.