Upgrade current laptop or snag a new MBP (but which one?)

I have a Macbook Pro 13" Mid-2009 2.26 Ghz Intel Core 2 Duo, 4 gb of Ram (used to be 8 until my logic board failed). I run my laptop with an external monitor maybe 80% of the time. Once you plug the monitor in, the system seems to start running quite a bit harder than usual. I assume this has something to do with an integrated graphics card. I had 8 gigs installed for many year until recently it seems one of my RAM slots died on me and my computer wouldn't boot. Once I fiddled with the RAM setup, I could get my computer to start up again but I seem to have lost an entire slot for RAM. The system still seemed fairly taxed even before the RAM slot failure - particularly if I ever listened to Pandora in a browser for any length of time.

So now, I want to continue running on an external monitor (maybe 2) but I'm also a bit worried my computer will fail even further. I'm an attorney and run some large PDFs (think 23k page PDFs), lots of word documents, omnifocus, evernote, lots of tabbed chrome windows (maybe 8 at a time with 15 tabs each), and just tons of windows in general (maybe 30 on average?)? Whenever I tell the Apple store reps that I'm running the system hard, they look at me like I'm a confused child and claim there's no way my setup could not handle everything I'm doing. But I feel like what I need is the best processor I can get with at least 8 gigs of RAM and possibly a discrete graphics card so that my CPU isn't trying to push pixels while it's also pushing the rest of my data (which seems to limit me to the current top of the line 15" MBP).

But I'm also open to upgrading my current MBP some more (although it sounds like I can't change out my logic board, so I'm not sure what I would do).

Any thoughts would be greatly appreciated.