It really depends on if you want to preserve OS 9 compatibility or not. Let's assume that you have a FW400 version that is OS 9 compatible and you want to run OS 9 on it also. If you do, then your big league video card options are the Radeon 9000 or the GeForce 4 Ti. The Ti is faster, but the 9000 can be bought in a 256MB VRAM version; I don't believe the Ti 4600 ever came with more than 128MB. I use a 9000 Pro with 256MB VRAM and it works fine in 9.2.2. No other options are fully compatible with OS 9 or are sufficiently performant.
As far as RAM, you might as well put 2GB in it, because RAM is still pretty cheap. Only OS X will see all 2GB; OS 9 will only see 1.5GB (technically it sees all 2GB but the system makes the other 512MB inaccessible).
OS 9 is not picky about internal DVD burners generally. Anything compatible with OS X should work with OS 9 in that respect.
Finally, the MDD can take an Encore/MDX CPU upgrade. Whether this is worth it to you depends on the work you do. I have dual 1.8GHz Sonnet 7447As in my MDD, and it runs like a bat out of hell in Virtual PC under 9.2.2, but CodeWarrior is mostly disk-bound and benefits little, and losing the L3 cache can hurt in some circumstances. It will not be cheap to find one. The 7447 is not compatible with versions of OS X prior to 10.3.5, but it will work with OS 9 with a firmware update (included).
If you only have the FW800 version, it won't boot OS 9. In that case, you can pick anything that works with Tiger. For that model, you might look at a later GeForce AGP, but my choice would be a Radeon 9800, which although it originally came with the AGP G5 should work with the MDD. Also see
http://www.barefeats.com/kwik2.html
