Can I increase my ram from 8gb to 16 gb

Here is a listing of the systems within the A1278 model: EveryMac A1278 list. As you can see its a big list! One of these is your system can you isolate out which?

As to upgrading RAM:

I do strongly recommend you first upgrade the firmware first follow this Apple TN on how to check and update: EFI & SMC firmware updates for Intel-based Macs.

As to performance improvement RAM is a good start, make sure you use two matched modules (same size, make and series) that way the system can interleaf the reads & writes across both modules. If you don't use a matched pair you can get into some system crashes more so when the RAM is more used. So don't waste your money with different sized or mis-matched modules.

The best improvement is upgrading your HD for a SSHD (hybrid drive). This gives you the performance of a SSD without the loss of the storage space of the HD. Besides I'm sure your running low in free space as many in this series had smaller HD when they were made. Seagate makes a nice one: Seagate SSHD. We have over 200 in the field now and have had great success with them.

Warning! All of the systems in this series have a SATA III (3.0 Gb/s) HD port. You need to replace the drive with the correct SATA I/O speed'ed HD (SSHD). Most of the current HD's are fixed at SATA III (6.0 Gb/s) and while they may appear to work they won't do so reliably. Make sure yours is able to run at SATA III (3.0 Gb/s). As an example review the Seagate SSHD spec sheet Note the line: SATA Transfer Rates Supported (Gb/s) 6.0/3.0/1.5. Make sure what ever drive you use as something similar offering 3.0 Gb/s.