The Xaser cases by thermaltake are excellent! Although to setup the thermal probe can sometimes be a pain in the butt and realize there will be a lot of wires running in there for you to setup so you have control over your fans and such like that.
If you get a RETAIL processor it'll come with a heatsink and fan, if you get an OEM processor it WILL NOT come with a heatsink or fan and also will not carry any sort of warrenty so your best bet is to get the RETAIL mainly for the warrenty. I'd suggest getting an aftermarket heatsink for the hell of it just because those processors are awfully powerful and even having the SLK-948U on this AMD64 system I built it still has temps in 50's so I don't even know what the stock would put. I am however going to reseat this particular heatsink because I may have put too much compound underneath. A heatsink such as that DOES NOT come with a fan so you will have to purchase a seperate one with that.
With a motherboard like ABit AV8-3rd Eye you get 6 channel audio and with the MSI Neo2-Platinum you get 7.1 channel Audio so a sound card isn't needed.
That graphics card you have is pretty good, however have you considered holding out for either the X800 or 6800 Cards? At this point I'm not going to argue which is better because it'll only invoke a endless pointless arguement over which is better. Truth be told, either one will freakin rock any game out there. ATI had some troubles in the beginning with drivers but it seems they are smoother now. Also the 6800GT card can be overclocked to the 6800 Ultra speeds without hardmodding or anything like that, simply requires some softmodding and isn't as power hungry. If you go that route get en eVGA Brand 6800GT as they put on the same heatsink on every card, meaning you get the heavy duty heatsink meant for the Ultra regardless if you even got the 6800 128mb card. Plus eVGA is a really good brand and seem like they focus heavily on making good graphics cards. I don't know a good ATI manufacturer though so I couldn't recommend any there.
Also the 6600 and x700 series should be coming out around christmas which should be around $200 and from what I've read will seem to be the most bang for your buck. I do know for sure that the $200 spent on that 6600 would probably beat out that 9800 Pro, although I've heard reports (from beedub) that his friends 9800 pro beat out his X800 but you gotta realize every system is different. Also the X600 series from ATI is garbage so don't waste your time with that.
Look into aftermarket heatsink and couple that with a good fan, vantec tornados produce a LOT of air movement but are extremely loud, you will hear that over anything.
9800SE's can be OC'd to a 9800 PRO level but you'll again need aftermarket cooling on that. If you look at ag3's signature you'll see he has an aftermarket heatsink on his card and that particular one is about $17.50 and again the best bang for your buck regarding cooling. The thing about the SE's is that they have the same core as the pro just has 4 disabled pipelines. When the 9800SE's are in high demand they simply take the 9800PRO core based chip and disable 4 pipelines which YOU can enable which will make it a pro, however...if those cards AREN'T in demand, then they take the 9800 PRO cards cores which are defective and have bunk pipelines and as long as it has 4 working pipelines they call it a 9800SE....problem is if you get one of those obviously you can't OC it to PRO.
So get: 9800PRO (possibly SE) - ATI
X700 - ATI
X800 - ATI
6600 - Nvidia
6800GT - Nvidia (specifically eVGA for better OC and stability)