My wifes still using her 2.26gh C2D 13MBP from 2009. She probably has similar usage to you, web (Facebook being the most taxing), videos, large iPhoto library, some web processing/etc. I only needed to replace the paltry 2GB it came with to 8GB.
I haven't even needed to switch the HDD, as from sleep it resumes almost instantly and consistently (as opposed to windows machines we have from the era we're sleep is unreliable and slow).
She's considering swapping for an rMBP for a couple of reasons now, but performance isn't one of them, as even a pre-unibody Mac she used while hers was with Apple was fast enough.
Firstly she gets student discount which brings the cost down and gives her 3 years Apple care. Secondly she's running out of HDD space, so considering throwing that money towards a new machine rather then an SSD upgrade. Thirdly she used my machine for a bit while I was arguing with Apple for a new battery (they agreed given only 300 cycles and second time it had happened). During that time she noticed (even after getting hers back with a new battery) mine lasted longer untethered and her pictures looked better on it.
This is the second time the battery has failed (expanded) and while Apple replaced it for free, she feels given the cost to upgrade (hers is still worth about £400-500 having canvassed people at work) and factoring the warranty she'll get it kind of makes sense to upgrade now. If she gets anywhere near that for her old one, I'll be astounded at the overall cost to her after 5 years of usage :O
I concur with others, almost any machine from the past 6 years will perform perfectly well for you. If budget is an issue, don't muddy the water by playing with the newer machines as its quite easy to spend twice as much when you'd be perfectly happy with a cheaper used machine.