You should measure your spacing - easy enough to do, just pull the wheel and stick a ruler in the dropouts or fork ends. Many 27" wheels are 120mm rear, many are 126mm rear - if it's a few off either way you should be fine. Are they steel rims? If you're not sure, just try sticking a magnet on. If they're aluminum, not too worn, and reasonably true, why not just replace the freewheel and save some money? If you do decide you do want new wheels, you have a few choices:

- new 27" wheels. If you have a good LBS they can order new wheels from QBP, I got a set of 27" Sun CR-18s with rear freewheel hub, 126mm rear/100 front spacing, for around $100 last year. They've been good enough for about 8-900 miles of loaded touring last summer and plenty of commuting on crappy streets, and are still true. I've also bought similar wheels from velomine.com for a good price. If you go that route you'll need a new freewheel too.

- used 27" wheels. Pretty easy to find if you're in a big cycling town, also plenty of them pop up on ebay. You can probably get a set for anywhere between $20-100, may come with a freewheel or may need a new one. Just make sure they're in ok shape before you buy.

- 700c conversion. You could get longer reach calipers for that (current brakes may or may not reach), but on top of getting a new wheelset (and new tires, too, I'm assuming), seems like more trouble than it's worth for a beater bike. Of course it could be a pretty easy little project for you to get your hands dirty working on this bike, too - nothing wrong with that.