Not a lot to go on there. Macs are allegedly picky when it comes to ram, that's why they charge a premium for it. (Or at least that's what they say.)
When the MBP does boot up, does it show the 2 gig stick, or only show 1 gig in total?
DDR 2 5200 ram is cheap these days. Best Buy and Office Depot have sales on the 2 gig sticks on a regular basis. BB: $42.99 for PNY 2gig sticks; OD: $24 for one gigs. Generally, they seem to alternate between ram discounts on their weekly ads.
Go to the apple guy at the best buy when he's not being swarmed, and ask him if the ram on sale this week will work in your rig. They're usually pretty good and honest on this subject (though they may look over their shoulder first to see if a manager is watching them).
Also, I believe your machine will only actually use 3.3 gigs of ram, even though it will see and recognize all 4 gigs. It's a hardware limitation, my 2.16 C2D white imac is the same.