For $500 you can get a good basic hybrid (7.2 FX, etc), which I think is a good style for your needs. The hybrids tend to be efficient and comfortable, with excellent utility for bike commuting needs (can accommodate racks, fenders, etc).
Rule #1 - FIT - make sure you dial in your fit. a poorly fitting bike is worthless at any price.
I think that having a little flexibility with price (as it seems you have) can be helpful in finding and buying the "right" bike for you. If you know what size you need then you might be able to find a great deal on Craigslist, or perhaps a carefully chosen bike from BikesDirect such as
this one.
Bear in mind that the bike is only part of the total cost of getting set up for commuting, but the added bits can be purchased/installed as time and money permit. These items (in order of priority, as I see it, coming into the summer months) can include:
Immediate/with bike purchase:
1. Front and rear "blinkie" lights for better visibility to traffic (especially in daytime). The brighter the better.
THIS video does a great job to demonstrate how invisible we can be. Blinkies help tremendously.
2. Repair basics: Basic multi-tool, patch kit, spare tube, pump (CO2 is for racers, IMO)
3. Saddle of handlebar bag for #2, or these can be stowed in your backpack/panniers
Helpful/Convenient to have soon (if not right away):
1. Rack/panniers OR bike-compatible backpack (ventilation, good weight distribution, etc)
2. Fenders - in Upstate New York, this go to the top of the list, IMO. Fenders make life so much better in rainy/slushy conditions
3. Rain gear (if you don't already have something that can fill the need). There is an excellent thread about jackets
HERE. Rain pants aren't really necessary (IMO) in the summer, but come in handy below 55-60F and also serve as an outer wind layer in the Winter.
Things to consider before October:
1. Good headlight
2. Studded tires
3. Bike-functional clothing - gloves, balaclava, etc
Finally, if you've been away from bikes for some time, take some time to focus on learning some basic maintenance (like fixing a flat and adjusting derailluers) and riding techniques (like avoiding "cross-chaining", cadence awareness, etc), which will help get you started in a good way.
There is probably a local co-op or shop that offers some basic bike maintenance courses.
Good luck!!!