It looks like you're talking about the "ready to ship" HP Pavilion Media Center a1310n PC:
CPU: AMD Athlon 64 3700+
HDD: 200GB 7200RPM
RAM: 2 x 512MB DDR400
CD: 16X DVD-RW
Price = $629.99, not including tax or shipping, but you do get a $50 mail-in rebate
If you wanted to build a similar system:
CPU: AMD Athlon 64 3700+ San Diego Socket 939 Retail
$215.7 (ZipZoomFly)
mainboard: Biostar GeForce6100-M9 mATX
$64.25 (newegg)
HDD: Western Digital 250GB SATA WD2500KS
$103.9 (ZipZoomFly)
RAM: Corsair 2 x 512MB DDR400
$78.99 (ZipZoomFly)
CD: NEC ND-3550A black DVD-R
$44.64 (newegg)
case: Cooler Master Centurion 541 mATX
$58.99 (newegg)
PSU: Xclio 450BL
$50.98 (newegg)
input: Microsoft black keyboard, mouse
$30 (newegg)
$647.45 including shipping
So, you'd probably save a little money by building it yourself (I seriously doubt that $50 rebate is going to make up for tax and shipping) and you'd get some better stuff. I'm not saying that's the best system to build for that price. Most people around here would at least spend another $10 to get a motherboard that can overclock. I'm just saying it's comparable.
If you don't want to hassle with building your own system, it's not like it's worth it in that price range. The difference barely matters. Seeing as you're the type of guy to post on a hardware forum, though, I can't imagine you'd be that put out by it.