5 years - yeah, it might be time. if there's one thing apple does 'well', it's finding a way to have everyone gracefully accept planned obsolescence. my old g4 iBook is still running fine for a friend, but my my real-use machines it's nice to have something still supported. that said, both my macbook and mini are prob not on the list for the next os release..... and it's incredibly annoying that both are just ONE step too far back for AirPlay!
Apple generally builds good quailty hardware - i've got much better spec Dell and Cisco stuff at work, but it's all low grade quality-wise next to apple consumer level gear. heck, if anyone has a god use for it, there's a Mac SE (yes, 1980's style) with *2* floppy drives PLUS hard drive all internal sitting a room or so away from me at the moment. all original, full working order - actually says made in the USA even.
for what you're looking for - assuming it's the image processing that's getting you down most recently - i'm with the guys voting for a mini. the quad core processors on the recent and current models do alot better than my old core 2 duo beast, and RAM is definitely your friend. 3rd party memory upgrades are VERY easy on the latest (no optical drive) mini chassis, and prices are excellent compared to what Apple charges from the factory. as for drives, sure, an SSD would be faster, but they;re not cheap if you want alot of space. I've upped the drive in my mini once already, and have a 750gb sitting on the desk for it if i get time later this week. i've yet to try anything with the new 'fusion drive', but if you're batch processing, cache type schemes are of questionable value in my experience, as you're dealing with much more data than the cache will hold.