I recently picked up a 400xm + Robo + NetMMC.
I chose 400 for the extra speed, which is never a bad thing if you can afford. The price differential is minimal, but not zero. I chose the xm b/c in my experience, 4M is a little small for libraries, customizations, etc.
I agree with Dave that unless you need WiFi and/or large storage that you can get from CF, I'd go with MMC. That's why I chose the NetMMC -- I get my MMC, plus RJ-45 for an easier and faster interface (vs. serial).
It all rang up at ~$300, with extras and shipping.
I'd get the 256M or 512M MMC from the site. You're not going to want to rely on your on-board Flash for everything, and popping the MMC into your host and moving stuff around is often faster and easier than transfering over serial or ethernet. Having MMC also makes flashing a whole lot easier and faster than over Kermit.
You'll want an MMC reader for your host machine. I picked one up at BestBuy for ~$12, but they're getting hard to find. CompUSA also has some really small ones for ~$20. Some work in linux, others do not -- Save your receipt.
You'll also want to buy the 5v adapter for your bench work, whether or not you use a battery pack for mobility.
You'll need a null-modem cable, which, if you don't have already, you can pick up anywhere. They're also available on the web store.
You'll also need an RS232-to-TTL(3.3v) converter. Check out (www.acroname.com) for one of those. Depending on your host, you may also need a USB-RS232 adapter.
It all connects up as such (for me):
USB -> USB-to-RS232 -> Null Modem -> RS232-to-TTL -> robostix.
All this serial stuff is necessary even if you get an RJ-45 unit. You need serial to flash (as far as I know), and if you're ethernet needs reconfiguring on the gum/robo, you need serial to get in there and change configurations and such.
I'd HIGHLY recommend perusing the wiki site for info on securing all the pieces of the stack together before firing it up. A minor wiggle or bump can fry these boards. Basically: some 0-80 screws, nylon stand-offs, and (optionally) washers.
Oh, one more thing, you'll need a LOT of patience...
Good Luck, and welcome to the community.