I do 3d graphics and am always low on hard drive space even with 2 big drives.
Can I add a 3rd data hard drive as a slave on the DVD drive cable without any performance issues?
Yes it will work fine.
You shouldn't have proformance issues at all.
Just make sure the cd rom is a master and the hard drive is a slave.
If you really want to go with something better you should consider getting a raid system. A new motherboard that can handle 4x raid hard drives and 2x ide.
There might be some performance issues when you are reading from the DVD drive and writing to the hard drive as the slave or vice versa.
You can buy an IDE add in card for about $15 from newegg.com, or an SATA add in card. Both would allow you to add 4+ harddrives/DVD drives easily.
OK thanks for your help.
Though with a 1 year old mother board ABIT KV8-MAX3 will filling up the card slots mess the system up?
I remember in older days that too many cards fought for interrupts. That is why I did't want to add the extra IDE card.
Also the 3rd Drive will be data only no programs.
if you are in windows xp with a board thats only 1 year old you should be fine, I had all my 5 slots filled on a P2 500mhz and I never had any problems.
I think the times worrying about IRQ assignments are over. 
well, if you have a PCI based video card it would fight with the SATA card for the PCI bus (degrade performanc), but I really doubt many people have PCI vid cards anymore.
Almost every add on has some performance trade off... lol.
