I like having a 4-core CPU so i can make use of netRender without needing a "net"
You'll want the 64-bit version of Windows so you can go past 4Gigs of RAM.
4Gigs worked fine for most stuff but 8Gigs has met all my needs so far.
A lot of mother boards just have two slots, and since the RAM works best in pairs you have to decide right away how much you're going to put in. But with a 4-slot motherboard you could fill two slots now and upgrade later with the other two slots and not have to toss out the RAM you started with.
My SSD has made booting and loading faster but my motherboard doesn't support its fastest mode (AHCI) so I'm not getting the full benefit.
I think some of the fastest SSDs now plug into one of the short PCI slots rather than use the SATA interface.
I have a 250GB SSD for my OS and programs and all my data is on a 3TB hard drive.
You'll probably want a large hard drive.
The best addition for using A:M is a second monitor. When I'm animating it is fabulous to be able to take the PWS and spread it out on a whole screen so I can see lots of key frames without needing to pan around all the time.
Make sure your video card can support two monitors.
Any cheap, used monitor will do but you want it to have the same vertical res as your primary monitor