Note that with a proxy, this only keeps a lightweight httpd tied up, assuming the page is small enough to fit in the buffers. If you are a busy internet site you always have some slow clients. This is a difficult thing to simulate in benchmark testing, though. Of course, ask around and avoid disks that have a reputation for headcrashes and other disasters. With money in hand you should think about getting a RAID system.
RAID is generally a box with many HDs.
It is capable of reading and writing data much faster, and is protected against disk failures. It does this by duplicating the same data over a number of disks, so if one fails, the RAID controller detects it and the data is still correct on the duplicated disks. You must think about RAID or similar systems if you have an enormous data set to serve. You need a fast disk controller. So either you should use the one embedded on your motherboard or you should plug a controller card if the one you have onboard is not good enough.