The new G4's

G4/867MHz Dual Processor 256MB RAM / 60GBHD / CDRW-DVD-ROM / GeForce 4 MX
G4/1000MHz Dual Processor 256MB RAM / 80GB HD / DVD-R / ATI 9000 Pro
G4/1250GMHz Dual Processor 512MB RAM / 120GB HD/DVD-R /A TI 9000 Pro
- All with 56K modem, 10/100/1000 Ethernet, 4x PCI, 2x USB, display cards as listed

have also shipped and I have installed several of these also. As these are all now dual processor machines they are all extremely fast (especially in OS X which supports multiple processors in all applications and the finder - unlike OS9). To give you a comparison, even the slowest 867MHz model is around 400% faster than the initial G4/400MHz single processor models initially released. An impressive list of new features and updates on several hardware fronts.
http://www.apple.com.au/powermac/

DDR RAM
This is good to have, as in theory it enables memory functions to work at twice the speed (data is transferred on both the rising and falling of the clock cycle - hence the name DDR : double data rate). In practice there is a lot of debate in the Mac technical community as to how well the new RAM is supported by the processor, motherboard and internal busses of the new G4. Bench testing indicates little if any speed improvement. There is no doubt that in theory DDR RAM is better than the previous SD RAM but even with faster system bus speeds this hasn't translated into noticeable improvements. Strange indeed. More to follow on this but Apple has been quiet on this issue.
http://www.barefeats.com/pmddr.html

G4's & Internal Zips
Internal Zip drives are a problem with the latest model G4's. I have fitted one, but it was an ugly thing to behold. Call for details and other options. Apple altered the drive bays and a good thing (in concept) but they didn't get it quite right. In previous G4's, Apple had a 3.5" bay custom designed to fit zip drives but almost nothing else. In the new G4's they have totally reversed the situation by having a 5.25" bay that fits all other CD's, DVD's, etc., but wont work with zip drives. Several problems exist - 1) their internal IDE cable doesn't have it's connectors placed correctly for allowance for connection to anything but a CD type mechanism, 2) they don't yet supply a facia bezel, 3) it is difficult to remove the existing swinging cover for that bay to fit another bezel and 4) you need to get brackets to fit a 3.5" device into a 5.25" bay. All of these issues can be solved but requires some minor ingenuity and is time consuming to setup. Apple really needs to fix their internal guts of the machine. They came so close to getting it right too.

G4 Graphics Performance
The new graphics cards however do give substantially more performance than the older cards in older G4's. Quartz Extreme in OS X 10.2 also adds hardware acceleration support to suitably new Macs with supported graphics cards.

Graph of tests by Apple at http://www.apple.com/macosx/jaguar/quartzextreme.html
- This compares all the new machines with the varying graphics cards available

Graph of tests by bare Feats at http://www.barefeats.com/pmddr4.html

Graph of various cards in an original G4 with a 16MB ATI display card
http://www.barefeats.com/sawgraf.html

As you can see, both the processor and the card installed makes a deal of difference. Interesting that the G4 is almost double the performance of the iMac with the same card installed. Is this the better motherboard and architecture or the dual processors at work? Probably some of both and shows the benefits of a G4 over an iMac or eMac. The greater the load, the better the top ends card perform in comparison.

ATI has also just released a new card - the Radeon 9700 Pro. This is not yet available for G4's, but expect it to make a big splash. This surpasses even the past king of performance the GeForce 4 Ti by typically 20-50% (although these results are for PC's not Macs, the results are expected to be similar).
http://www17.tomshardware.com/graphic/02q3/020819/radeon9700-11.html
http://www.anandtech.com/video/showdoc.html?i=1656&p=18

In particular, the large 22" & 23" Apple LCD's seem to benefit greatly from having at least a GeForce 4 MX installed. You can also order GeForce 4 Ti cards to give the ultimate in performance (for a price)... until the Radeon 9700 Pro gets released anyway.

Summary of Graphics Results
1) G4 towers have almost double the graphics performance of the G4 iMac/eMac
2) ATI Radeon 9000 is about 15% faster than the GeForce 4 MX in the same G4
3) GeForce 4 Ti is about 50% faster than the ATI Radeon 9000 in the same G4
4) Original G4's (with 2x AGP) are bandwidth limited and will benefit only minimally by upgrading the graphics card. Put in a relatively cheap GeForce 2 MX or GeForce 4 MX.

For your edification, the fastest Pentium 4's running the latest graphics cards can achieve frame rates of over 300fps. This is 50% faster than the fastest Macs using the fastest graphics cards and have support for up to 8x AGP (4x in G4 Macs).