Well i'm looking at your system and in truth i see quite a nice system, not cutting edge admittedly but still modern and purposeful.
It's all going to come down to your objective, Do you want to play games at a higher resolution? Do you want to encode quicker? Do you simply want that feeling of being 'current'? (I hope not, because that last one will keep costing you
).
If you do upgrade, my advice is to limit your expectations, because you'll be spending relatively large amounts of money for relatively little benefit. I'm generalising slightly here.
Overall, if it's just about games you'll be better off (financially) just changing your graphics card alone.
That all said, here are my recommendation at the best price/performance components in the market currently:
Case: Your call, but something by Antec rocks.
PSU: Corsair, around the 500W area.
CPU: Intel quad cores, lower end (the price/performance curve goes nuts here the higher you go)
RAM: I always buy Corsair, but 4 Gig of DDR2 is an easy recommendation for any OS. (Anyone who mentions DDR3 has forgotten what living on planet earth is like).
Motherboard: The Intel chipset P35 series has ruled for 6 months already, the P45 will be out soon. Either of those. Abit, ASUS and Gigabyte all make good ones.
Graphics card: Anything from the £80 Radeon 3850 up to the £170 Geforce 8800GTX all hit their price/performance curve, and anyone who tells you that x is better than y isn't considering the money. X may be 20% better than y, but costs 20% more. Below that is false economy. The other factor is what you are upgrading from, and in truth a 3850 will only be a bit (50%?) better than your current card, whereas a GTX will be what... 300% better? I'm broadly guessing here. Graphics cards devalue quicker than any other component though, so the more you spend the more you lose. And the next series is about 1 month away, so don't rush right yet.
Hope that helps