Some people like smartfan. If you have an Asus board, it has software that monitors everything. Your bios tells you whats going on also. There are others.
Judging by your system I would say its the ram because you processor and video card are definitely not the problem. Maybe you should invest in a stick of 512 if you are playing some newer stuff like doom 3.
Motherboard Monitor 5, sorry I don't have a link, but if you just type it in google it'll be like the first ones there. Also good utilities are CPU-Z and WCPUID but the last two basically give you the same information. You'd be able to tell if you had a bottleneck with CPU-Z because it'll tell you if your ram and cpu have a 1:1 ratio. If it doesn't then you need to adjust it to make it 1:1 and thus ridding yourself of a bottleneck