In addition to the previous comments, not all chainwheels are usable on all setups. The reason is that the little pulley system hanging from the rear derailer is designed to take up the slack when you're in the small front chainring. If there's too much difference in the number of teeth between the biggest and smallest chainring, the derailer won't be able to take up all the slack. Also, the front derailer on a mountain bike probably can't tolerate a 52-tooth chainring.
A better option would probably be to change some of the cogs in the rear. By replacing the rear cogs and going with a small gear with fewer teeth, you'll get some higher gear ratios. You'll want to keep the largest rear gear the same unless you never use the biggest rear gear with the smallest front gear (ie, "granny gear"). You'll have to replace all the rear gears to keep a consistent spacing.
Here's a great reference - check the chart at the bottom to see some acceptable combinations of teeth for a 7-speed (that's what you have - 7 gears in the rear). You'll need one of the wider gear combinations to get you something that you can use for uphills on trails while keeping you from spinning on pavement, toward the bottom of the chart.
This probably isn't beginner maintenance, but a good shop can help you. One further bit of clarification - the above assumes you're using your bike on trails and pavement. If you're only riding on pavement...while I appreciate you can't afford a *new* bike, I bet you could afford a *used* one. Craigslist has a ton of cheap-o bikes that may well be cheaper than the surgery you're talking about.
EDIT: Sorry for the dupe, started writing the above before Sheldon's post.