Do you even need the X38 features (dual PCI-E 2.0 with XFire and uhhh ...?). It might overclock better, but most if it's features can be found on the P35. I suggest you get a Gigabyte DS3L (~90$) or DS3R (~120$, if you need the extra SATA ports and/or RAID support). Put the money you save on a GTS to replace the GT.
I also support mihirkula first 3 suggestions (HD, cooler and PSU) and his statement about 4GB for Vista.
I run Vista 64 and I have no more issues than I had with any other OS. The games might run a bit slower on Vista, but I never really cared that much about losing 2-3%. I lost 5% on 98SE -> 2000, 2000->XP, XP->Vista; my guess is that I will lose 5% going from Vista to Windows 7 ...
The main attractions of the P5E to me are the pci 2.0 support (the future proofing) and the seperate sound card that comes with it. Pricewise I could save € 11,- (in Europe) when buying the Gigabyte. I know that's like a gazillion US dollars right now, but what with the extra soundcard I'd have to say the P5E is the way to go. But, correct me when I'm wrong (or you are more right).
The advice on the harddrive is spot on though. I'll definitely switch to the Seagate.
The same goed for the cooler.
As for the power supply: I allways wonder who gets to say something is tier such and so. I can only assume he is far more knowledgeable than I am. Consider it switched.
I'll have to think about the 64bit Vista.
Thanks again for your helpful replies! I'll keep you posted on how they will work out for me.