Once games are loaded into memory, your CPU and graphics processor will have the largest impact on performance.
The amount of RAM in your system plays no part in speed of your system, unless you are already paging to disk. If you're not, and since you can't upgrade your CPU or graphics processor, the SSD will give you your best overall performance boost.
In your position, I would do the SSD upgrade first, and then upgrade the RAM later.
Your games will love you for the SSD upgrade because textures, bitmaps, etc. will load into memory off the SSD much, much faster. This will translate into you getting in game quicker, levels loading faster, etc. if you're looking for better frame rates, then you'd have to upgrade CPU/graphics or play at lower resolution or lower quality settings in the game.