I have Deep Fritz 8. I’m running it on a 1.73 GHz, 2 Gb RAM Windows laptop. I am a patzer. I use Fritz mainly for game analysis, with the occasional game in Friend Mode.
Is there any compelling reason for me to get Fritz 12 or Rybka 4?
no
Fritz 12 comes with 1 year of premium access to playchess.com.
Premium access gives you free access to grandmaster commentary of major chess events in real time (match Anand-Topalov comes to mind as a good example). Others have to pay money, each time they want to access this kind of stuff. So, if you can get Fritz 12 reasonably cheap, say below $40.00, it is great deal.
Otherwise your ancient Deep Fritz 8.0 should be fully adequate to your needs.
The Rybka 3 or 4 with Aquarium GUI is very nice. I prefer it for game analysis and in depth positional analysis. I like the way it displays alternative moves and their relative value in the game.
I do have Fritz 12. That interface has taken a little time to learn as it is different from prior versions. But, the things I particularly like about Fritz are the friend and sparring modes for training.
Are these compelling? If you can only opt for one, go with Aquarium. You won’t be disappointed.