What is the difference between 80mm 90mm and 120mm besides the obvious fact of size? Is it a significant difference between 80 and 90?

The most important aspect of the fan size differences is what mounting methods does your box provide? If it has space for 120mm fans, the smaller ones will require "engineering" to mount. Check your box's specs at the manufacturer's site, if possible. It will tell you what fan sizes it accomodates.

Bigger fans can push more air at once, and therefore can usually run more slowly and quietly for the same amount of cooling power. So bigger is generally better, assuming it'll fit in the case.

You should also take care to buy high-quality, quiet-running fans. A cheap one will probably drive you nuts pretty quickly. Built-in variable-speed control is a plus.

Here's a good rundown of quiet fans:
http://www.silentpcreview.com/article63-page1.html

80mm is the one for the case im looking at

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Get a case with 120mm mount openings. You'll have more airflow with a good quality larger fan... and less noise.

dang and i finaly thought that i found the right case but if you guys say so ill get a 120mm thks

It's all a matter of preference. My case has only 80 mm fans, but has several quiet ones. You should weigh several factors in selecting a case. My top 2 factors are accessibility (I like to be able to remove drives easily, look for one with screwless drive mounting) and airflow (what you're looking at).

THIS is my Antec Super Lanboy case, with 120mm fans. It also has slide out disk trays, an excellent feature! I love this case. Brushed aluminum, see-through plexiglass left side, storage compartment, Three 5.25" and two 3.5" standard openings, etc. All specs HERE. Cheap too!

I can also recommend Antec. They seem to put a lot of effort into designing cases that are both quiet and have good airflow for cooling. I put my new system in an Antec Solo case, and it's pretty sweet. One big 120mm fan (even the stock fan is very quiet) in back, and spaces in front for two more 92mm fans. I installed a couple of nice Scythe fans tuned to slow speed, along with a good quiet power supply and a mostly-passive cpu cooling system, and the thing is very hard to hear unless you get really close.

Their Performance 180 also gets very high marks. Review here.

The super lanboy is not that great an option. First of all, it's made of aluminum, that's great for portability, but not good for noise. Also, it's getting to be a very old design, and antec has revised its cases a lot since then. The P180 was very impressive when it came out, but since then it has fallen out of favour compared to the more modest P150(although there have been horrible issues with the 150's bundled power supply). There is now also an HTPC case with a design based on that of the P180, if you're interested in that.

How big a fan you need can depend on various things, if you only had a cool running CPU, like a single core winchester or venice athlon, you'd probably be fine with a 80 or 92 mm exhaust fan. Noise in general is also dependant on how far your computer is from you, and whether there's anything in between you and it.

This isn't true, at least among the quiet computing crowd. Both the P150 and P180 have been popular; choosing between them has always been largely a matter of personal preference.

Personally, I was never very impressed with the P180, but there's no denying its popularity.

I cut out the "grid" so that the air flows more freely. (Do this with an empty case.) I also tied cables away from heat sensitive components so that air can get at them. I remove any components that I no longer use, dial-up modem, floppy drives, etc. No point in generating unnecessary heat.

Personal preference can tip the scales in favour of either, but the P150 does have more going for it. It's smaller and lighter, the soft silicone grommets aren't as good at isolating the hard drive vibration as suspension, it's cheaper if you are fine with the power supply or manage to sell it for a decent price, and it doesn't have issues with the lenght of the motherboard power cable.

Speaking of silent PCs: SeaSonic makes some of the quietest fanned PSUs out there. Antec even uses SeaSonics in its cases. (Exception to the SLK3000/Bs, those were someone else.)

I'd go with the right size fan. If you can find a quiet one, consider a 120mm, but you don't need it unless you do a lot of work that would need that sort of airflow. Some people even leave the case fans out and then the air flow through the holes.