Use the link in my sig to edit you're first post. That will help with a lot of other questions.
But in general, if you don't have that much to spend, a Q9550 will be a big upgrade for you and will cost $220.
A whole new platform will cost more. AMD won't be considered here cuz they can't offer anything better than what the Q9550 can do and it would cost more needing a mobo.
And i5 750 would be big gains for you. The i7 860 would be bigger gains as that chip will hyper thread. With the i7 860 performing just as well as the i7 920, I'd just stick with the 860 on the P55 mobo. The X58 cost more and only gains you graphics performance, which you don't need.
CPU: i5 750 $200
Mobo: P55 $100-$135 (micro and middle road Asrock vs full sized and Gigabyte/Asus)
Ram: DDR3 4gigs $90. If you run a number of apps at the same time, 8gigs might be worth considering, but only heavy intense programs, not chat/music/internet apps. I'd start with 4 and see how the performance is, then if you need a little boost, more might be worth it.
GPU: The 9600Gt will still serve your needs
PSU: ? if its a decent one, keep using it
Case: ? if its worked this long, should still work in your next build
HDD: ? are you set
Optical: ? are you set
OS: ? are you set
The i7 860 would just be $90 more but use the same parts at the i5 750.
I was looking at the 229€ MSI P55 GD 80 but figured it diddn't offer anything more to me that i need - the cheap Gigabyte board seems more than enough for my needs and it should OC easily to 3.8Ghz (i hope)