Its not like RAM is crazy expensive. Well, it is if Apple's selling it. But I can't think of a good reason to not get as much as you can afford.
The best possible thing you can get, if you can work it in, is an SSD. Its the biggest improvement to the PC since discrete graphics. Spinning disks just suck as they age. If you need big storage, get a 2.5 inch external drive and call it good.