G4 Cpu Cache and Speed, what to go for

Some tests have shown that the older 7455A 1.2Ghz -1.6Ghz G4s, 256k L2 cache with a 2MB L3 cache will work better in older macs like ours, due to the limited 100Mhz bus. The 1.6Ghz with the dual cache will most likely work better on your system in some ways, the L3 cache makes up for the slow bus, but the 1.8Ghz model with the 512k L2 cache is based on the new low voltage 7447A chips used in the current Powerbook G4 has a little more punch overall. Now the 7447A does have a faster bus speed (167mhz) , but you cannot take advantage of it since your hardware is limited to a 100mhz bus. IMO, considering the age of your machine, limited bus speed, and cost, the 1.6Ghz may be the way to go.

If you want to play Doom etc, you'll most likely have to upgrade your video card also, if you still have the original Rage 128Pro or a Radeon 7000.

http://www.macsales.com/ is a good place to go for older Mac upgrades. As for the CPU, look at what they have to offer, although I recommend the Sonnet upgrades (they come in models of 1Ghz, 1.4Ghz, and 1.8Ghz on that site although there are other choices) myself since thats what I purchased over a year and half ago to upgrade my aging Power Mac G4.

CPU upgrade list...
http://eshop.macsales.com/MyOWC/Upgrades.cfm?model=148&type=Processor&TI=%23Time Format%28Now%28%29%2C&shoupgrds=ShowUpgrades
The choice you make is more of a cost consideration more than anything else when it comes to upgrades of this nature, and how long you are going to keep using the machine.

If you can aford it, its fine. The 1.8Ghz with the 512k L2 cache will be fast, no doubt about it. Most likely 2-3 times faster than your current 400, 466 or dual 500Mhz machine. Lets put it this way, you'll gain a lot of speed, and had at least two years to the life of your machine (till there are no up to date, or new apps that run on PPC).

Any of the upgrades will be fast. I moved from a 400Mhz G4 on my machine to a 1.2Ghz model with the 2MB L3 cache (which the 1.8Ghz and some 1.7Ghz models do not have) and I'm very happy with it. I play games like Call of Duty on my machine with no trouble at all, with a ATI Radeon 8500. A high end CPU will preform better, have better frame rates too and when I got it a year and half ago it cost as the current 1.8Ghz model (bad timing on my part I guess since prices seem to have dropped a lot with the second and then third generation G5s out now).

As for buying an upgrade you can order from OWC, they ship to the Netherlands.

OWC has a benchmarks page for comparing G4 upgrades.
http://eshop.macsales.com/benchmark/agp.cfm