As far as the RAM is concerned, you should perhaps just get the minimum RAM from Apple and buy apple-certified RAM somewhere else.
Apple are known for selling their RAM for a very high price, and it's particularly visible for big amounts (2*2Gigs). Their prices have dropped recently compared to a few years ago, but I still would be surprised if you couldn't save some money this way...
But do people using software loopers here need 4 Gigs of RAM ?
I've got 2 Gigs on my macbook, my PC at work has 2 Gigs too and they both work very well (I work in videogames development, with many applications needing lots of resources)...
The VST effects/synths mainly use the processors, and even big samplers or long loops won't use gigabytes of memory, or am I missing something ?
You can also have an external firewire hard drive and use it to record everything. This works quite well and allows you to have more flexibility...