Depends on your budget and needs right? Do you ever run out of memory? For almost everybody 8GB memory is plenty.
But. I have Late 2011 on order. I know 4gb is insufficient. 16gb of RAM was only $79, so I figured as long as it was open the extra $40 is no big deal and I'll never need to go in there again.
For SSD - since you already have your laptop you should know your storage needs by now. If 256GB just isn't cutting it now then you may have to spring for 512 or whatever, or plan to do some thinking about which apps you need to have installed and how to offload large files if you need to.
I would prefer 512G space but I prefer the 128G price. So I got a 256G Crucial M4 as a compromise. 256GB should be fine for MY needs, but I know 128GB would be difficult to maintain, especially after a year or so. I could have justified 512GB and in a year or so might be kicking myself but we'll see.
The prices are dropping so fast and I figure the 256G to last a few years. I'll put the 512G spinny disk on the optical bay for storage, and I already had an external disk for backups/storage. I got a DVD enclosure to put the optical drive into.
I sort of expect 1TB SSD's to be affordable in a few years in which case I can upgrade then, or maybe I'll be ready for a whole new system.
I intend to blog post about the whole upgrade process and I'll post a link to that when I get er done.