Computer Lag...CPU problem?

Hello,

Back in March, I upgraded my outdated computer to fit requirements for next-gen gaming, high-def videos, better support and less lag on Photoshop (as an artist, I need a computer that will not always crash/lag these programs) and similar programs, and just smoother use all around. At first I was pleased with the results, but once I started really using it, I noticed an extreme lag/low FPS/choppy game play while playing my video games. Not only that, but concerning my specs my computer shouldn't be this choppy/slow as it has been. It cannot even play videos (even low quality) correctly or smoothly. The computer I used to use for gaming/video viewing works much, much better and its specs are much older.

I understand a computer lags at times and such, but I would think my computer could handle things better. I don't have the best system, but as I said, the older computer works much more smoothly in some aspects.

I've gone through almost every old trick in the book: updating drivers, double checking if things are properly inserted or installed, etc. I’ve tested out numerous games, and they are not doing well at all. (Mass Effect, Assassin’s Creed, Prince of Persia, Guild Wars, Oblivion, so forth) I'm not new to building computers but there are a few things that I'm trying to figure out. Here are my specs:

NForce 730a Motherboard
Windows XP (SP3)
AMD Athlon 64 X2 Dual Core Processor 6000+ 3.10 GHz
2.00 GB of RAM
NVIDIA GeForce 9600 GT
(I had 4GB, but it was only showing 3GBs so I took one stick out)

I recently upgraded my motherboard (used to be an ASUS M2N68-AM) because a friend of mine said that board could not handle my 125 watt processor. Overall I am pleased with the new motherboard, as it is a better quality one, but it did not improve anything.

The next thing I suddenly thought of is a slight overheating issue. See, when I first start up my computer in the morning, and let’s say I right away play a HD video or game, it’ll be fine for a few minutes. After awhile, it starts to lag or become impossible to play, and even if I restart the application, it does not play as smoothly as before. Studying my temperature levels, it says my CPU is normally around 50-60 when I start up the computer and so forth, but if I begin to do anything that requires the CPU, it jumps to 75 at the most; though I have not caught it go past 80 yet. I was wondering, could I be having a fan that’s not cooling the processor enough, and causing some problems, or does heating really affect your computer performance?

I am almost certain it is not the video card, because I used it in the previous machine and it always ran fine.

I really would like to get this problem solved, since this has been around since March. If you have any advice, it would be greatly appreciated.