I was thinking about buying the HP EX470 and I have two unused Seagate 750gig EIDE drives that I would like to use internally. I was wondering if there is room inside the EX470 to use an EIDE to SATA adpator and if so, what are your recommendations for an adaptor?

No, IMHO there is not room. Without extensive modifications, you would need to do away with a motherboard back plate.

The drive fits in a plastic tray, the back of the drive extends out of the tray, (there is a small plastic flap that folds down when the drive is mounted in the tray), allowing the end of the drive to plug into a back plate.

Thanks for the info.

Put them in USB enclosures, and connect them via 2 of the USB ports

ahhh...good point....I was thinking about doing the same thing on some unused EIDE draive I have...but am I looking at significant decrease in speed if I do this?

In absolute terms, yes. USB 2.0 is measurable slower than SATA II

However, users here have reported streaming DVD's (Blu-Ray if I remember the post) from them without performance issues.

Give it a try.