windows 2000 pro should be fine as it supports up to 2 cpu's so it would see your dual-core cpu as 2 separate cpu's, as i have a hyperthreading single-core p4 windows sees it as 2 separate cpu's
I've used Win2K on my old PC, and I'd personally recommend the upgrade to WinXP Pro you won't look back.
I'm not sure if the dual-processor support in WinXP Pro is better than Win2K but it wouldn't surprise me.
However, if you're not a gamer then you probably won't notice the advantages of Windows XP over 2K as much as I did (compatibility etc). I presume you don't really run any non-gaming apps that require Windows XP (eg Cubase SX)?