I've got an ex-dual 533 DA myself. Great machine!
I would avoid the OEM Apple parts, since the only upgrade you could do would be to use another DA CPU card - but you already have the fastest DA they made! (I'm assuming you have the dual 533). You could put in a 667MHz or 733MHz CPU but they are actually slower than the dual 533s (they use a different G4 CPU that is less efficient than the 7410s used in the dual 533).
I would also avoid using a CPU from a Quicksilver - you have to do some modifications and risk ruining the CPU and/or your motherboard if something goes wrong.
My recommendation would be a third party CPU upgrade. Like za9ra22 said, OWC is the place to go. I bought OWC's own 1.4GHz upgrade for $300, and it makes a huge performance difference - especially if you upgrade the hard drive and video card at the same time.
Another upgrade I'd recommend is Power Logix's 2.0 GHz upgrade for $369. Even though it is 600MHz faster, the speed difference isn't all that huge due to the fact that the OWC chips have L2 and L3 cache, while the Power Logix only have L2. The OWC units are also more reliable, if the xlr8yourmac.com testimonials are any guide.
But there are a ton of possibilities to choose from. Even at the bottom end you can get a 1GHz upgrade for under $200, which is the same or only slightly more than you'd pay for an Apple Quicksilver OEM card that will require soldering and praying.
Good luck!