I'm going to assume that your dual 450 is a Gigabit Ethernet, as I don't think that particular configuration came in anything else.
The nice thing about these is that they can take 2gb of RAM(some later G4 towers deleted one RAM slot and thus maxed at 1.5gb). Get 4x512mb sticks-they're cheap now, and will really help the performance of the computer.
The GPU is-unfortunately-somewhat limited due to the 2x AGP slot in this system. Fortunately, if the computer is in base-model form still, it likely has a Rage 128, and virtually any card you install will be an improvement. If you want to run OS X exclusively-and in particular Tiger or Leopard-you will greatly benefit from a card that supports Core Image. Offhand, the only cards I know of that will work in a 2x slot are the nVidia 6200 and the Radeon 9800. The 6200 is only available as a flashed card.
If you want to retain the ability to boot OS 9 natively(i.e. not in classic mode) you don't a Core Image card as you will have no hardware acceleration. In that case, I'd suggest either a GeForce 4MX or a Radeon 9000. Both of these are roughly equivalent in performance. They are also inexpensive and readily available. They also work decent under OS X. Just as a caution, if you go with the 9000, be sure to install the correct extensions for it in OS 9 before installing the card. The GeForce 4Ti is the "king" of OS 9 graphics cards, although it's tough to find and can be expensive.
If your single 400 is a Sawtooth(AGP Graphics) all of the above also applies to it.
If, however, the single 400 is a Yikes!(PCI Graphics) your options are a bit more limited.
First of all, you can only put 1gb of RAM in these. 512mb sticks work perfectly fine, but are read as 256mb.
For the GPU in a Yikes, your "official" options above the Rage 128 that most shipped with are the Radeon 7000 and the Radeon 9200. The 7000 is generally easy to find inexpensively and will give a significant performance advantage over the Rage 128. The Radeon 9200 is tough to find(at least the Mac version) but is a great card.
Alternatively, a flashed GEForce 5200 will give you Core Image support. I have one Yikes! set up with this card along with the matching graphite 21" Trinitron CRT-this is a great combination.