my friend speaks the truth. computes detect whichever has the fastest read/write speed between the HDD and the RAM. 99% of the time the RAM will win. the 1% is only for REALLY old pc's when they used to have 180mhz "SDRAM" which (somehow) was slower than a 3200rpm hard drive... that stuff was shocking... now with your DDR3 ram, you're looking at transfer speeds even an SSD couldnt compete with.When you start a program, much of it is loaded into the RAM. So unless the HDD you're using for storage is extremely, extremely slow (worse than a 5400rpm laptop drive ) then I won't make much of a difference, except for the initial loading.
the SSD will help speed in 2 areas, initial boot up (before the computer will load any programs/data to the ram at all) and in initial loading of HUGE files. ie files larger than the capacity of your 4gb of ram. so if you have a 5+gb iso file, or a 1080p movie thats <5gb... other than that, your hard drive will read the file and transfer it into your ram for quick usage. once the program has been closed, it will unload from the ram, and the cycle continues with the next program you run.
for the price of an SSD, do note what differences it will make for you... shaving seconds off a boot time is all well and good but its quite a cost for it in a price/storage space ratio...
not saying don't buy one, im more saying lighten the boot load of your OS before presuming you need one