I have a VPS with 30 WordPress sites. I am running nginx & APC and notice it eats up my RAM up to 1GB and it increases each time I add another site. I notice APC is the one that's causing the increasing of ram usage. Is there a way to optimize this?
It's hard to answer this given that each site is most likely different and each server is also configured differently.
If these sites are individual WordPress installs then 1GB /30 sites is normal, an absolute bare minimum per site would be 32MB for
32MB is in my opinion way to low, it's impossible to tell from your question how loaded the sites are but you should be looking at having more RAM allocated to APC, this would exceed 1GB of RAM easily.
You should look over the APC settings, this is like fine tuning a car, it can make a big difference or destroy your server so be careful.
Some things that will effect your RAM use:
ps. Do not play with APC settings on a production server.
I am using for caching Redis for wordpress.I got amazing results.