My guess is:
- maybe minor speed bumps in the near term. I think this is unlikely, though.
- the big update will be ~October -- for the Christmas season.
I'd really try to avoid getting a C2D computer at this point. The Core i5/7 processors are really pretty nice.
OP, you will breath a lot of new life into your old iMac with a RAM upgrade. With only 512MB, there's a good chance those beach balls are due to VM swapping to/from disk. Even 1GB would make a huge difference.