Commuting - Which brands/models of bikes should I be looking at?

I'm a 21 year old male, 240lbs and 5'11". I'm out of shape right now for sure.

I also lost my driver license recently for 90 days. I will be commuting to work 18 miles a day round trip.

I have a Gary Fisher "Mamba" Bike from about 8 years ago but its very heavy and I get out of breath fast.

I'd like a full suspension bike with mountainish-style tires but a front suspension would be fine.

The weight factor is huge, I'm looking for ideas of what brands to look into that are a good bang for the buck.

Budget is around $600 I'm thinking...give or take. This is how much I spend in gas in about 4 months just to get to work.

You will get out of breath with any bike if you are aout of shape, I would keep going with the old one till you are _sure_ that you really like commuting and you will not give up, and then spend the bucks.

If you can also ask friends for test rides of other bike styles you should be able to determine your ideal ride, you are complaining of a heavy bike, but you want mountain-bike wheels... when the wheels are usually much more noticeable in the biking effort than the 5 kg you save using a fancy lightweight bike.

Gary Fisher "Mamba" Bike from about 8 years ago

Follow glacierre's advice. Quick fix that should make a difference on your Gary Fischer to hold you over for the 3 months, is to get narrower slicks (no knobs on tire ) for commuting and keep the tires at max. It'll be much easier to commute and minimal expense until you decide what to do.

Is there a reason that you want/need mountain tires?

I think you would be pleasantly surprised how much of a difference a set of 1.5" road tires would help with your commute.

Full suspension is absolutely useless for commuting unless you do some serious trails along the way, front suspension is close second. I don't have any suspension on any of my bikes because good suspension cost a lot.
Since you are commuting, no point of spending any money to suspension forks, check the commuting bicycle pics in the sticky, you will see very small amount of bikes with suspensions.

Your Gary Fisher Mamba is fine, just change the tire into some at most 1.5" wide.
Although I do have a racing bike that's pretty modern, I commute around town using a Trek Aluminum hardtail from 1992, the only modification is the slicks and fenders. That's it, and it's a $50 investment.

By the way, I'm 20 years old.

I like to add that for bikes with front suspension at $600 price point, some people actually switch the front suspension fork with a rigid one for commuting.

Here's what you need,

1.5" Slicks, look for Schwalbe Marathons, Tioga City Slickers (that's why I run), Continental Contact, etc.
Full fenders, from SKS, Planet Bike, if where you live constantly rains and snow. Don't go for the half-assed mudguards, not as useless as full fenders.
Lights if you are riding at night.

You can spend up to $100 for these at most.

I would put slicks and fenders on the Gary Fisher, Also weight make almost no difference to me. My bike weighs 48 pounds with all my junk and stuff on it.

+1 to what everyone else has said.

Do not invest too heavily until you are sure that you will stick with it.

However, if it snows in your area I would stick with knobbies for the time being.

yep, decent road tires will make the most difference. add your 240 to the weight of the bike and even getting a bike that's 5 pounds lighter won't make much difference. what will make a difference is riding it for a few weeks. you'll be stronger, thinner and feel a hell of a lot better. fenders, lights and a good saddle (brooks!) will also help. imo the suspension is only making that bike harder to ride. best of luck, let us know how you feel after a few weeks!

SSEiYah,
How far is your commute and what are the roads like? Do you need to change your clothes upon arrival at work? Will your commute take place in dark, snow or fog?

I absolutely agree with the precious posts. And, run your current bike up to your local LBS (local bike store) for a tune up to make sure it's in good shape. Also, do you have tools, a spare tube and patches in case you get a flat (and maybe a headlamp if it's dark)?

not to repeat but, slicks will make a work of difference I got the forte road slicks from perfromance for like $8 eack plus $3 each tube and it make the biggest difference, night and day. make a good DIY headlight and you got a commuter. all for under $100