S.L.
We had my husband's 50th at the Sunnyvale Community Center. There are plenty of other Community Centers in the San Jose area that might rent a room cheaper. We had a large circle of friends at the time, so a half dozen of the ladies decorated for me, another friend DJ'd and we did a potluck for the food.
For his 45th, we reserved the common room in my condo complex. If you know anyone who lives in a condo who could reserve their clubhouse for you, that would be a less expensive option. Time Deli on Bascom and West San Carlos has done catering for several events in our social circle and does a nice job. That's an option if you have more of a budjet to work with.
We've also had several birthday parties at Bucca di Beppo in the Pruneyard. The food is served family style and the place is pretty noisy already, so they don't mind a loud crowd.
HTH.
S.