Just got an x51 - have some basic, basic questions as a non-gamer (details inside).

Hello all.  After a catastrophic computer crash, my well meaning in-laws (who know less about computer hardware than I do) bought my wife and I a new x51 machine with the following specs;

i7-3770
6GB DDR3 memory
1TB Serial ATA hard drive
NVIDIA GeForce GT 545

The only thing my in-laws really knew was that we wanted somehing small, because we spent a lot of time complaining about how big the last computer was, and how much space it took up - and based on the size alone, the x51 is perfect. But neither my wife or I are gamers - this computer will be used mostly as a productivity machine (office work - photoshop, word, excel, ppt, etc.) and media streamer (streaming 5tb of video, some of it in hd, to various xbox360's hooked up to hdtv's around the house).  Having said that, my wife can get REALLY into starcraft and warcraft (not WOW) every so often, and has spent countless hours (days, months?) playing those.  I haven't looked at hardware in maybe 6 years (when I bought the last computer - the one that just died). So I'm hoping some of you regulars might be able to help me with some basic help.

I think that machine will serve our current needs as is, considering it's replacing a 6 year old machine with 4gb of memory, but can someone confirm that?

If I wanted to upgrade the memory to 8gb, how difficult would that be?  How do I know if I need 2 1GB sticks or a 2GB stick or whatever?  I saw that dell offers a 2GB stick for $21.99 here.  Is that the one I would want? How difficult is it to install?  Do I just pop it in?

Like I said, we're not really gamers, so I think we'll be ok with the video card (confirmation on that please? All I'm seeing is how outdated the card is for gaming, but what if you don't really game?) but if I wanted to upgrade the video card to the 660 that the current x51's are getting what would be involved with that?  I saw some threads on upgrading the video card, but those all looked like they were upgrading to something a little more advanced than the 660.  I know that I would need a new power brick, but were do I get the actual card?  I don't see one from dell, and plugging NVIDIA GTX 660 into amazon or newegg gives me a ton of results - which is the right one?  And would it be as simple as just popping the old card out and popping the new one in?  I think I saw that in a youtube video.

Finally, the hard drive. I've never used a SSD before, so I don't think I'll miss it, but other than loading times, does it really make that much of a difference?

So yeah - thanks in advance for any help, and please no comments on weather or not my in-laws got ripped off, or how out of touch I am regarding hardware, or how I could have built the perfect computer for probably half of what they paid, etc.  I found a lot of that from various forums I was checking out trying to find these answers.  Thanks again.