Is this a good idea?

I want to connect all my devices on a single molex line (DVD-RW, X800, 2 HD's). The cabling would be nice and neat but I'm wondering if each device drops the voltage a bit. Should I split into 2 lines coming from the PSU (I have a Neopower) for better performance?

yes - dont stick em all on a single line - the x800 would be better on a dedicated line - but certainly dont put em all on one

agreed. i got a antec TP550. the videocard is on its own cord. and the hdd is on its own (well..beeing that it comes with a sata hookup with the psu.. lol)

but.. i got ..my cdrw and my cooldrive on one. ..and a i got one leg for just fans.. and somehow its still very clean

First of all, all the fans go on their own leg, seperate from any equipment.

I run my harddrives on one leg

my vid card with my cd-rom (I am not going to be using cd-rom and gaming at same time) and floppy (when was the last time I ever used one of those?). So my graphics card is essentially on its own line because I never use the drives. This keeps wiring neat, and gives enough power to the devices that need it.