Processor or HDD?

I have a 4 year old MacBook 13" i7, replaced with 8GB RAM. I use it everyday while filming using a screen grab system. I import HD video and then can tap or hold a button to capture a screen grab, sometimes up to 5 a second. Recently every time I start capturing the fan goes mad, much to the annoyance of the sound recordist!! I asked an apple salesman and once I told him that I was operating up to within 6BG left on my HDD he said my HDD was too full and needed to empty it. I freed roughly 100GB but it wasn't long until it would happen again (i had only used up a further 10GB before it started again). I have noticed as well that while using it with HD video it does slow up at times and can get quite hot. Is it just my HDD getting over used or is it the processor? I have been reccomended to replace the HDD with a solid state drive, if this is the best route which drive is the best, or is getting a new MacBook (quad core) the better route. Any help appreciated.