MacBook :: System With Most Recent OS Incredibly Slow?

My wife has a Macbook (2.16 GHz Intel Core 2 Duo - Mid 2007) that has become painfully slow. We brought it into the Apple store just before her Apple Care expired and they replaced her hard drive, as they said there were some issues with it. I maxed out her RAM last night to 2GB (and yes, the machine is recognizing the RAM just fine). I also upgraded to Snow Leopard this morning. Other than that, there hasn't been any work done to the computer. She uses it for very low intensity things and doesn't do much more than surf the internet and load photos into iPhoto. That said, the HD only has about 5GB of free space. Now I know it is nearly impossible to diagnose any problems with the machine through a message board like this, but I am wondering if anyone can offer any insight into the most common reasons a not-too-old computer with a new HD, 2GB of RAM, and the most recent OS would be incredibly slow.

I keep her Activity Monitor open in the dock so she can see how much of her memory is being used, and it's kind of interesting to see. Upon start up (with the new memory installed) about 3/4 of it is free. After opening just about any program it shoots down to about half free, and similarly, upon opening another program (it doesn't seem to matter which one) she has about 1/4 of her memory free. Of course, closing down the programs free up some of the memory, but it doesn't seem to get back to that 3/4 that were there upon start up. I usually have a decent grasp on what is going on with a computer, but when it comes to something like this I don't really know where to turn (especially without Apple Care to eat the cost of a fix). So, anyone have any thoughts on what I should do to get the machine running at a relatively quick pace again?