a large portion of your choice will be on where you want to install your application, if you plan on installing *just* the OS on the 25 gig partition, i would say you would be fine, this would allow you room for updates and SPs. you would then create a second partition to install your applications. you would want both your apps and Vista installed on the same partition i would go for something larger, currently i use 120 for the primary OS partition plus applications. since you are using a 64 bit OS i would allow Vista to set the pagefile size. it does a repectable job and can adjust it self as needed.