There's no compelling reason to get AppleCare immediately, just don't forget to buy it before the one year warranty expires. It's not hard to find AppleCare discounted 15-25% off the suggested price of $249.
$150 for a 1G stick mentioned above is about 90% higher than necessary. Corsair Value Select from Newegg works well, about $80 per gig.
Best deal all-around is probably to order from a place online, with $200 rebate, cheap shipping, and no sales tax. For example, Amazon.com. The cheap and excellent Dell 24" goes for about $900. Add it up, you get to about $4500. Settling for a 20" Dell screen brings things down another $450.
To fit that budget, with an Apple display (they're very pretty), you'd need to get the 20"