"What is the difference between 2.4GHz and 2.8GHz? Does it matter?"
It's the processor speed, the faster machine will feel a little snappier. Unless you're heavily into gaming (in which case I'd caution against getting a Mac) or video processing then the different probably isn't going to be that noticeable.
"What is a solid state drive?"
It's a lump of electronic bits with no moving parts. So, in theory, it's much quicker than a traditional hard disk drive (such a drive has to move a little read/write head into position to get at each bit of the disk, this slows it down) and also much more reliable (moving mechanical bits are more likely to go wrong).
But such drives are traditionally more expensive, although there is meant to be a bit of a drought coming up for traditional disks and this might force them to be priced above solid state disks.
"My current laptop is full to bursting and has 320GB storage so I need much more space for pictures and music"
The more space the better in my opinion.
But remember that you'll need an external disk drive that's pretty much as big as your computers disk drive to back all your data up to, don't just have it in one place, that's asking for a failure to cause it all to be lost.
"Will it connect to my wireless internet/ printer as easily as my pc does?'
Wireless internet should be a yes, there are standard ways of doing such things that the Mac plays well with.
The printer is worth checking, the manufacturers web site should have some information on that piece of the puzzle.
"Will I be able to get it to talk to my desktop which uses Ubuntu?"
Define "talk"? You should be able to set up folders on either machine that the other one can access or have a big drive plugged in somewhere that both machines can see. Underneath the hood the Mac and Ubuntu actually share an awful lot in common, both are based on the same technologies, although Apple have wrapped theirs up in a very very nice shiny shell.
"Do I really need a Macbook?"
I think they're great, but they don't make cheap entry level Macs. They're well built and well equipped machines that are in my opinion a delight to use. I had my previous Mac for about 6 years and only replaced it when I broke it.
So for "casual" computing that includes a chunk of music and video I think they're a very good choice especially as you don't have to worry about viruses and security.
However...
For gamers they're a very bad choice, there aren't many games available for them.
For people who like to tinker with their machines they're a bad choice, Macs are hard to upgrade and a lot of the "it just works" experience is down to the fact that you can't rebuild them and throw whatever bits you want into the box.
Personally I also don't like writing software on them, Microsoft's Windows toolkits trump Apple hands down pretty much right across the board. But that said Apple do throw in their software development tools for free and you'll pay a lot of money for the equivalent from Microsoft.