[KarmaKiller]

Cheapest Quad on egg:
Newegg.com - Intel Core 2 Quad Q6600 Kentsfield 2.4GHz 2 x 4MB L2 Cache LGA 775 Quad-Core Processor - Processors - Desktops
Same one I run. They solid CPUs.
Newegg.com - Intel Core2 Quad Q9400 2.66GHz 6MB L2 Cache LGA 775 95W Quad-Core Processor - Processors - Desktops
That would be the next Quad (as far as price goes) that I would get.

And on the upgraded later. In theory, any CPU you pick up can most likely be upgraded. BUT, is it always worth the price is a whole other question. Intel already has the newer socket out (1366) and are already in the phase of switching over to that socket. Now I'm not saying this PC is gonna be slow or anything. But the chances of the upgrade 2 years from now actually being worth it are pretty slim.

As far as the HDD, this is my normal suggestion:
Newegg.com - Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
I just picked one of these up myself, and they are fast. If you want more then that, then I would suggest:
Newegg.com - Western Digital Caviar Black WD1001FALS 1TB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
But they aren't as fast as the 640gb drives.