T110 Memory upgrade 2+2+4+4?

Hey I have a Dell T110 server (I'm pretty sure it's the first version - any easy way to check) running Ubuntu Server x64.
I was wondering about upgrading the RAM and have two questions:
1.
Where should I buy it from? Dell seems to be charging thousands of dollars for the RAM (probably a glitch), Crucial has some Dell T110 RAM: http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=7F...
Which is a lot cheaper, should I get this stuff?
2.
Currently I have 2x2GB sticks of RAM (4GB) is it ok if I add 2x4GB (8GB) sticks in the remaining slots?? Or should I remove the previous RAM? I'm running a Minecraft Server with some leaky mods so I do need a decent bit of memory  .

Thanks in advance!