I've never owned a piggy before but I did do some research a while ago when I was interested in getting one.
I do know that guinea pigs need allot of space. You really shouldn’t keep them in any of the cages you find in the pet store, most are two small. I’ve seen that popular cages are these home made cavy cages.. here is a link to pictures of them
http://imageevent.com/cavyspirit/cavyca ... ecoroplast
You custom build them how you like, but as you can see they are quite large.
I also know that Guinea pigs are social animals and require another guinea pig to house with. So getting a piggy means you need to get two. On top of that that are apparently very demanding for attention and need to be taken out quite a bit for play and exercise.
And of course count in any vet expenses. I suggest doing allot of research before you find a pair.