MacBook Pro :: Relative Power Of The 13" Vs 15" And Older?

I tried making a thread in the buying advice forum which only garnered one response, so I figured this forum would be of more help since it appears more frequented and the visitors would be more specific in their knowledge. I can also be a bit more precise in terms of questions. I have a 13" MacBook from late 2006, which I believe was the very first line they came out with. It's running a 2.0Ghz Core 2 Duo with 2GB of RAM (personally upgraded; that's also a lot of twos). It's been a good machine and I've had it for three and a half years now; AppleCare has run out and it's already starting to show wear from when I had the casing replaced for a second time (that wristpad plastic still cracks no matter what Apple does to fix it) as well as the fans and logic board when it was showing issues. The machine is old, though, and going on four years doesn't have as much processing power as I'd like.

I'm not a real power user any more. I gave up trying to learn about all those things like Linux and programming in my mid-teens and now I'm just in my early 20s and more of a writer and casual gamer sort. I don't play anything more graphically intensive than WoW, or perhaps StarCraft 2 when it goes retail. I own a perhaps three year old desktop now that belonged to my friend, but was given to me when he replaced it not long ago. It runs most of these games pretty well, so I do have the option of using that machine if I need to. My question is if the power of the new machines -- the 13" in particular -- is enough to do what I want them to do while still making me feel like I've made a substantial upgrade for what I've paid. I know the base MBP is running a C2D. It's faster than what I have, but I don't know how much faster, and I imagine that the i5 in the 15" is likely far superior. I also can't really get a definite opinion on the power of the 320M which is also in the baseline MacBook. Either way, if I upgraded, it would be to the MBP solely because of the ligher, higher quality aluminum enclosure. I just don't know if I should shell out $500 more for the 15", for the reasons above. Thank you in advance for any opinions you might have.