The best place to purchase condos in my opinion is close to job centers. San Ramon, Pleasanton, Dublin, Danville, Walnut Creek, and Silicon Valley are all good places to look. What you are going to find is a trade off between strong rental market and yeilds. Closer in markets like the ones mentioned above are easier to rent. However, prices are also higher, and it becomes more difficult to break even with 25% down. Most of the time it seems to take around 40% to break even on condos in the Pleasanton, San Ramon, Dublin area. You will be mostly banking on appreciation.
Further out in the central valley (Tracy, Modesto, etc) you can get better cash flow, and you will get more of your return today in the form of positive cash flow. Future appreciation may not be as certain in outlying areas, but you can find arguments on both sides of that one.
One thing to be very aware of is the financial condition of the HOA for any condo you purchase. Some of the areas that have been decimated by foreclosures have condos with associations that are in terrible financial condition, and there is risk in that scenario. Many of these associations may see sharp increases in the HOA dues to make up for deferred maintenance and legal fees as a result of so many of the units being in default. As always, find an expert agent to help you.
Here is a link to good information on the San Ramon area in particular http://www.680homes.com/san-ramon-real-estate/
The 680 group at Alain PInel Realtors