You got really excellent advice here so read all the posts well then make your decision.

My opinion for the computer would go along the same lines as the Cateye Mity 8 or if you want to save more money the Sigma Sport BC800 is cheaper by about $10 each and has all the basic functions-a good starting computer.

Performance has a floor pump that cost $20 when on sale OR you could buy one of those $20 electric air pumps from a auto supply store or Walmart, then you kill 2 birds with 1 stone because you could use it to pump a car tire up if needed; then just buy a brass schrader to presta converter for a buck and leave it attached to one of your bikes wheels so as not to lose it.

Gloves I use Trico Sports gel 1/2 finger that cost me $15 and way out lasted the Pearl Izumi gel-lite that cost $35!

As far as what is needed that varies, I also commute on the bike and like to bike to remote areas so I carry a expandable rear seat wedge bag made by Road Gear that cost $15 and has a reflective strip around it, this is where I have 2 mini tools-one from Park MTB1 that is a bike specific too and a Eddie Bauer with pair of needle nose pliers and some other tools I got from Target for $10 and it's real small. I also carry in the seat bag a dozen glueless patches, new tube in its box to protect it, a ultralight folding Torelli tire, tire irons (plastic will prevent marring the rims), a thing called the VAR I got from Rivendell that makes installing the last section of tough tires a snap without any possiblity of damaging the tire, tube or rim, spare batteries for tail light and computer, ID with blood type and phone number (also write your blood type inside your helmet), $20.50 for food or whatever and maybe phone, phone card (in case phone won't work with coin), mini first aid kit-not much use for major problems but will handle small scrapes and cuts, I know I'm leaving out some stuff but can't remember what. BUT this stuff you can add piece at a time as money allows and/or importance dictates.

You also need a pump to take with you, either a frame pump like the Zefal HPX or a mini like the Torelli Aria which is what I use and it attaches to the side of one of your bottle cages.

I just take a backpack when commuting for my lunch and shoes; I don't take a change of clothes because I only live 3 miles (now only 1 mile since I moved) and I ride a slow pace in the morning so no sweat issues. The ride at night is when I detour about 12-15 miles home. You can get low cost backpacks from Walmart or Target, but try to find one that has a reflective beading on it's back.

If your commuting your also going to need a light system of sorts, you will find this key to your survival!! The headlight can either be one of two types-self contained batteries or a seperate battery pack. Cateye EL300 is the brightest of the self contained and cost about $35 but for $10 more you can get the Cygo Metro (I have this one) that has 12.5 watts total power based on a dual beam light-6.2 watt flood and a 6.3 spot and it runs off of 6 D batteries that last 5 hours on one beam and 2 1/2 on both if you run it continously. It totally blows away the Cateye 300 with just one beam! And last season my batteries lasted 4 months of commuting. I get a 12 pack of D's from Walmart called EverActive for $8. Taillight the best is the Cateye TL-TD600 and cost about $20, also theres a new taillight that you mount one in each end of each side of your handlebars and it's extremily bright and cost $20 for the 2. I use both and I have had people come up to me in there cars and remarked how well they could see me from far away even.

You can get pant straps that have a reflector built into them for about $10, they will help you even more to be seen. get the widest you can find.

Also get a helmet-if you haven't already. Any bike helmet will work, even the cheap $20 ones at Walmart, they all now have to pass the same regulations as even the expensive ones called the CPSC certification. The expensive helmets have better air flow and better "cool" factor.