Reverse Osmosis filter

I'm in the market for an RO filtration system. I can afford anything really, but would prefer to get something between $100-$150. I use like 10-20 gals a week, 20 would be more than enough for now, so I don't need a huge pump.

I'm not very educated on types, I just know how they work. What are the differences between different stages, like 5 stage RO filter, 6 stage? What do I want to look for in an RO filter (brand)? Also, is there any common stores such as lowes or sears that carry a reputable RO system that I could simply go pick up? I'm avoiding buying one online, but here's what an ebay search came up with.

http://shop.ebay.com/i.html?_nkw=reverse+osmosis+filter+system&_sacat=0&_odkw=reverse+osmosis+filter&_osacat=0&_trksid=p3286.c0.m270.l1313

Are there any filters in there which seem to be a good buy?

Sorry for all the questions, but I'm just trying to get educated. Thanks.

Learn all about it as you build a custom unit.
I did a 4 stage with pressure gauge and a few extras for $160
including delivery.

0.5ppm every usage, works perfectly.

http://www.bulkreefsupply.com/store/build-your-own-ro-system.html

Hope that helps

hydrofarms stealth 100 RO system works for me, only had it for a week. so far so good

www.purewaterclub.com r/o's for under $100

Any inputs on this?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130388673291&ssPageName=STRK:MEWAX:IT#rpdId

I think i'm going with this one, cheap price, 4 stage, 75 GPD is fine, very small (and portable). I live in an apartment and don't want a big unit, and can't install it anywhere. This one looks solid and should get the job done.