Expanding my gaming PC

Is that a trick question? You want to add something to improve it, but not replace anything? I am not sure how you would improve it without replacing anything. You pretty much have everything you need. Making case mods may or may not be considered improvements, as what you do with case is all in the "the eye of the beholder" What 1 person thinks is cool, the next will think completely sucks. I am of the opinion that a good looking or functioning case should be left alone. If it's not running hot, you don't need more fans (simply more noise) Water cooling is a waste if you are not a huge overclocker.
You do not want to replace components, but you asked for opinions, here is mine.
You need a faster processor. If you are gaming, and do not overclock, get an e8500. If, and ONLY if you play with Flight Simulators, get a q9550.

Wow. Someone runs a Q6600 at stock speeds? Really? No, really?

It,s a nice system they only thing i could think of that you could add is a SSD HD but if I were you I would wait untill they fix some of the current issues with random long acces times, That's what i,m waiting for. As evongugg said a new gpu like the 4870 might be nice but you could also wait until the HD5*** comes out and get one then.

As for water cooling I would not really recommend it. I used to have it on my pc but it's just really expencive and will only benefit you if you overclock what i did,nt do and you aswell. So a good air cooling will work just as well for less money.

col-p-todd - Do you know when the HD5*** range is coming out? Before Christmas per chance?

Why in the world would you add a water cooling if you are not OCing? The only thing you could do to improve your performance noticably is a faster C2D or C2Q 45nm cpu and upgrade the GPU to a factory OCed HD4870 or 260GTX or 280GTX or either flavor of X2 cards beginning with HD 4850x2. I would have to say that your refusal to OC your system makes your question a bit superfluous as more performance would result from OCing your present CPU and maybe selecting a faster GPU as the 8800GTS 512 (although still a decent GPU) is now a bit dated performancewise.