Having more RAM is no the same as a SSD drive but it gets pretty close. Once a program is open and then closed/quit that program will reside in the Inactive RAM (blue in the attached file). It will stay there unless you need that RAm for something else but assuming you don't then after the first time you open the program it will take only a second or so. If using editing software like Photoshop the RAM will be first used as scratch and if more is needed it will use the hard drive as scratch. With an SSD that scratch space is like a RAM boost and will help.
However, if you are reading plenty of information - like scrolling through a huge image library, the speed of access that information is related to the speed/density of the drive. So a 750GB 7200 RPM WD scorpio black (the fastest HHD laptop drive today) will be significantly faster than a stock 320gb 5400rpm drive, but also much slower than a faster SSD.
If you need plenty of storage space on your laptop then max out RAM to 8GB (about $100) and get a large fast drive like WD. If you definitely do not need plenty of storage then a 120GB SSD might be better. The 2011 MBP can use 16GB of ram but that will cost about $1600 from OWC. A much better choice would be a 500GB OWC SSD and 8GB RAM which will cost roughly $1,000 and offer much better performance. Personally, I upgraded my 2010 MBP with 8GB ram and a large WD 750 scorpio black to hold my large photo library and those upgrade cost a little over $200.