There's no reason on earth to ever put 8GB of RAM in a 13" with integrated graphics and screen real estate that small. 1) It's extremely cost-prohibitive. I'm sure you can find it for about 1/2 this price from a third party (I personally only trust RAM from Apple but that's me) but from Apple isn't the 8GB RAM like a $1000+ cost right now? 2) it's unwise because you'll never use it. RAM allows the computer to do more at once. Maybe if you have a 24" monitor hooked up to it...then, maybe, but I highly doubt that's your plan. ...so it's just a spec. But, we're spec people. We're impressed by specs. When a laptop had 32mb of RAM we were awed...then 64, then 128, 256, 512, 1, 2, 4...I have 4 in my 15" Pro unibody and I'm amazed and now you can 8! I do a lot--but i don't do enough to even push the 4 to the limit. I could survive with 2 -- easily and I'm doing media stuff. So, on "potential" alone do not make your decision.
Things to factor in...
1) front size bus and RAM speed of your Pro - if it's a 2007 it's either 667mhz RAM or 800mhz -- and I'm thinking the Pro had 800mhz FSB. The new Pros have 1067mhz RAM/Bus. In my opinion this means the internals of your computer aren't causing so much bottlenecking to a very fast processor that can do a whole lot of stuff.
WINNER: 13"
2) screen size - not a huge issue but 13 isn't 15
WINNER: 15"
3) video card - on the one hand you have what 256 dedicated video ram in your 15" --but in a video card known for failing. The 9400 has 256 but it's shared and not as powerful. Course if your 8600GT video card has lasted this long with no issue maybe you got a good one.
WINNER: TIE
4) connectivity. In my opinion the 15" has the better option offering an ExpressCard slot and a DVI out. I think the 13" wins in the display options - now with Mini Display Port to HDMI adapters available (I saw one the other day online for about $15) and easy access (Apple Story) to MDP-->VGA OR DVI the display out on the 13" wins. It's nice to have Firewire 400 + 800 but it's the additional USB ports that lead me to believe your current set up wins here
WINNER: 15"
Bottom line - if you were upgrading to a 15" Pro with an Express Card Slot and a dedicated video card this is a no brainer. I don't see a real upgrade here going to a 13". I've seen refurbished MBPs with the 9600 card and Express Card slots for under $1400 on Apple.com- that's your winner.