Unless you're doing number crunching or something, I'd probably pass on the X2 right now. Maybe next year.
If it were me:
* Fastest 939 pin Athlon64 for < $200.
* ~ $100 Asus motherboard (maybe the A8N-E) - can take an A64X2
* At least 1 GB, maybe 2 GB of RAM (~ $65 to ~ $170)
* At least 250 GB, 7200 rpm hard drive (~ $100)
* LiteOn DVD burner (the last Plextor I bought has problems) ~ $40
* < $75 PCI Express x 16 video card
* Antec case ~ $100
* Misc case fans, etc. $25
~ $700 + shipping. More, of course, if you want a new monitor, speakers, keyboard, mouse, etc., etc. It's not a screamer compared to the latest and greatest, but it'll be plenty quick (especially compared to an old machine) unless you're one of those online gamer people.
Browse around [link|http://anandtech.com/guides/|Anandtech's Guides] to see their view of the latest hardware.
HTH a bit.
Cheers,
Scott.