I would upgrade that CPU to a Core 2 Quad Q9550 for $169.99 from Micro Center (may be $190, not sure), pickup only though. Then if possible try to get a 64-bit version of Windows 7, otherwise a better graphics card like an HD 5770 would be good as well.
Since he mentioned Canadian currency, it's safe to bet he can't get any microcenter deals.
You didn't mention what your using your computer for, but if your gaming I would get a better video card first. A 7900 is very low end and power hungry for todays standards, after that I would consider getting a Core2Quad and overclocking it if you can get a good deal on one.
ye its for gaming but why a quad core ...isnt dual cores better for gaming
Actually, most current games can utilize up to 3 CPU cores. Some, like Dragon age, FSX and BFBC2 can use more than 4 cores. Older games (before 2007) can only use 2 cores.
But if you are gaming, I think your CPU isn't great, but it is sufficient. It performs like the Athlon IIx2 240, which was a great budget CPU before the x3 s were released.
I would upgrade the graphics card. I know I wouldn't be happy with anything below my current radeon 3870.
Ever ordered from NCIX.com? They have great deals. http://ncix.com/
Again, Lmeow is right about 64-bit windows 7. Right now I don't think it should be your priority for gaming because between 3-4GB is the current ram "sweetspot" and there are not yet any 64-bit exclusive games. But you will want to look into it in the future.
Make your upgrade after this one a CPU upgrade, along with a motherboard and ram upgrade if you can- so you can get something not on socket 775. If not, a core 2 quad would do well. But is not needed yet.
If you would consider overclocking, you would be able to eliminate any potential bottleneck with your CPU, but I doubt there will be much, if any, in most games.