Since you will have 2 separate drives, it is possible to install Linux on your new mSATA SSD without touching anything on your main 2.5 inch HDD. To prevent any possibility of screwing up, simply put the mSATA SSD in, remove the 2.5 inch HDD, and install Linux as if it's your only OS. When the installation is complete, put your HDD back. You can then choose which OS to boot by pressing F12 at the BIOS splash screen and pick your boot device. (It's also possible to chainload Linux from the Windows boot menu.)
As for which distribution... I don't like newer versions of Ubuntu because of their new Unity desktop. Consider Kubuntu (Ubuntu with KDE desktop), or maybe Fedora.
X60 tablet 6363-P3U, 3GB ram, 128GB SanDisk Extreme SSD, SXGA+ screen, Intel 6300
T61 Frankenpad in 15 inch T60 body, UXGA LED-lit AFFS LCD, T9300, 6GB RAM, Intel Graphics, Intel 6205, 128GB Crucial M4 SSD, 1TB HGST HDD + eBay caddy in Ultrabay
701c butterfly, 75MHz 486DX4, 40MB ram, 1GB CF card