just adding memory won't speed up your 'puter indefinitely
must people running vista can enjoy 2 or 3 Gb - if the version is 32 bit ... you'll hardly get any benefit from the 4th Gb because the 4Gb limit includes every scrap of memory on your machine.
SSD will speed up your machine - because they are considerably faster than a standard HDD
on older machines adding ram willy nilly may actually slow them down.
all that said ... unless you bought a very budget machine
most makers want their machines to look good ... so given any particular OS the machine will be configured properly.
faster tends to be measured in tiny steps
so a p4 3Ghz machine is only slightly slower than a 3.6Ghz machine ... hardly noticeable at all.
a p4 is noticeably faster than a p3.
my quad core blah bla will recode a 2 hour movie in a quarter of the time it takes my p4 (as you'd expect)
if it's been ok and suddenly starts being slow ... it could be a fault
but more likely you need to do a bit of housekeeping
http://www.ccleaner.com/download
and a defrag can work wonders