Games won't use anywhere near that much memory; they are only 32-bit.
For general multitasking and gaming, 8GB is sufficient, 16GB is plenty. I can have a 4GB VM, several Chrome tabs, PDF reader, NetBeans, a few other misc programs and a game running all at once and still not use all my 8GB and everything alt-tabs instantly.
Imaging/desktop publishing would benefit from more RAM--how much these days, I don't know; perhaps someone with recent experience can chime in on that if you do those sorts of things.
Additionally, you'll have to check your motherboard manual and your OS for limitations (Win7 Home Prem max is 16GB).
RAM would be low on the list to give your PC longevity and, after 10 years, the platform would have changed once or twice at least, not to mention CPU architecture, host bus changes (especially video card interface), etc.
If you're a gamer, the video card is the top priority.
As for RAM-picking, I like Mushkin. They deliver top speeds without increasing voltage and they are priced very well. The only downside is you may have to hunt a little to find some that aren't backordered. I don't care for Crucial or Corsair (I've had too many RMAs with those guys) and definitely not OCZ (junk..if they're even still in business).