First you should make sure the systems firmware is upto date. Follow this Apple T/N: EFI and SMC firmware updates for Intel-based Macs.
As to the system RAM your system can support upto 8 GB (two 4 GB modules) Not sure how you have 5 GB (one 1 GB and one 4 GB) If that is the correct, you'll still want to replace the older 4 GB module with a matched set of two 4 GB modules as this system doesn't like mixed RAS/CAS from two different modules of the same size.
As to pushing the systems storage:
Remember most of the problems people encounter is too full a drive and/or the drive has become fragmented. I would recommend you try using a good defrag app like Drive Genius. It's my goto tool for slow HD's first, you may find it fixes things!
OK, thats not going to do it. Then think about going with a SSHD hybrid drive like Seagate SSHD (9.5 mm height) instead it offers the zip of a SSD on booting up, yet still offers the deep storage of a transitional HD. This is our current direction on upgrading our laptops. Yes, I'm sure we'll be moving over to SSD's next year but for now this is the most economical solution for us.
Going with a SSD is still workable but the OS-X version you are using or planning on using could be a factor. TRIM enabler won't work with Yosemite without turning off driver signing which could put your system at risk. The older OS's don't offer signing so it will work with them. Some drives have housecleaning onboard so the need for TRIM is lessened. I would look at what OWC offers for SSD's. Also, don't forget the cheaper SSD's are smaller (240 GB). You'll need to leave about 1/4 to 1/3 of the drive space free. Where does that leave you given what you currently have on your HD today? The investment of a larger SSD may not be worth it here given the age of your system. Apple now uses M.2 blade SSD's so your SSD won't fit internally on the newer MacBook Pro's.
Just do the math out before you leap: How long you expect to hang on to your system and how much space do I really need.