My first disk (same type) became inaccessible after 3 months and I had it replaced with a new one. I wouldn't like to have the same problem. Can an enclosure damage a hard disk and make it inaccessible or the problem was with the disk itself?
The 640GB HDD should work. Enclosures can damage HDDs, usually from overheating. I like aluminum enclosures (one even has a small fan), and I leave them sitting on top of my case when I'm using them. Avoid placing the enclosure in a desk drawer or other closed space with no air circulation.