Not sure why Linux doesn't support that ethernet right out of the box at this moment. Maybe they are new or something.
The PCI version should be supported by kernel 2.6.13 (which is the next kernel), but I don't know anything about the PCI-e version, which is what I beleive you have.
There are drivers for it aviable..
http://www.syskonnect.de/syskonnect/support/driver/htm/sk9elin.htm
But you'd have to probably compile them yourself, which means that you have to have the headers for your paticular kernel installed and know what to do with the ./configure ; make ; make install type things.. which as a newbie isn't going to be easy.
But all that's what I've seen from a quick google around.
At the moment probably the easiest thing to do would be to just disable the onboard stuff and buy a cheap PCI ethernet card.
Just do a search for them in newegg with "linux" keyword or whatever you like to do. A cheap Zonet card is like 5 bucks, but a gigabit card is around eleven. Then you can get everything installed and such and worry about the marvell stuff when you feel like learning a bit on how to compile stuff.
A extra nic card is nice and handy to have around anyways, especially if you want to turn a old computer into a router/firewall or whatnot.