As I play more games, I realize how much more hardware demanding they have become. Before I get to the questions, here is a rundown of my specs.

2 GB DDR(PC3200) RAM
AMD Athlon 64 3700+ CPU ~2.2 Ghz
Geforce 9600 GT 512 MB
500 Watt PSU
Asustek Amberine M Motherboard

I have a widescreen monitor with a native resolution of 1440x900 and im running all of this on Windows Vista Home Edition 32 bit OS(I wish I would of stuck with XP by the way, Vista is a buggy piece of shit and it hogs too much memory.) Thanks to the Corsair website, I was able to select my PC from a list(since it's prebuilt and not custom) in order to determine what type of RAM would fit into my system. So around X-Mas time I plan to update to around 3-4 GB of RAM. My first question is this: Would shelling out the extra money for 4 GB of RAM instead of 3 GB be worth it? I know that games today recommend that there be 1 GB for XP or 2 GB of RAM if you have Vista or higher, even though this isn't necessarily an accurate indicator of how well it would run. I was thinking that 4GB is pretty much over kill these days and 3 GB would be just right, but im not positive on this. Im content with my 9600 GT(Even though I wont be able to max some games out or even come close, it runs very well for the money and I have no intents to upgrade atm.) But lets get to my CPU, which is still the stock CPU and I know that it is bottlenecking the performance of my other components. So the question is how to determine what CPU I will be able to slide into my motherboard? Quick Google searches on my motherboard make and model were fruitless so maybe ill have a little more luck here. Last but definitely not least, is a 500W PSU sufficient given that I want to upgrade my RAM and CPU? I dont want to upgrade only to be stuck with an inadequate amount of wattage at my disposal. Seeing as how im not the type to want all the bells and whistles in games to be content, this will most likely be my last upgrade for a while (school will keep me more than busy beginning next school year.) Thanks for any advice/recommendations beforehand!