I'm back. It looks as if my fears were unfounded.
I am running stable at FSB=140 (1.8A) @ 2.52GHz, memory (512Mb XMS 3000C2 v1.1) at 4:5 (ie 175MHz), settings 2-3-3-6. CPU Voltage 1.60, Memory Voltage 2.65. Standard cooling. P4B533-V.
Haven't had time to upgrade BIOS, am using version 2. Maybe an upgrade will also help, will see.
It looks as if my 1.8A is now the bottleneck, doesn't like going over FSB=140, even with higher voltage. Maybe better cooling would work, but I will leave it at this (knew I should have bought a 2.26B!!!). Will wait for P4 prices to drop, then get one.
Get memory errors if change settings to 2-2-2-5 or anything tighter than 2-3-3-6. In spite of that believe that memory will run somewhat faster at 2.5-3-3-6 but is constrained by CPU ability and 4:5 memory ratio.
So I need a 2.26B and Asus to fix their BIOS to give 3:4 memory ratio above FSB=132Mhz (fat chance!)
martn10, as I understand it the two stick problem is a BIOS problem by ASUS, and one can only assume that this will go away as new BIOS's are released. I don't think that buying a different brand of memory will necessarily help. Other brands may be more cost-effective than Corsair, and work just as well, but I can't advise you on that.
Something else - I did some tests, and it is quite clear that the memory performance drops dramatically if you use the onboard graphics. This is because it shares the main system memory, so it is better to use an external graphics card.
I don't know what you want to achieve, but if memory performance is important get a 2.26B CPU (for the extra bang on the FSB speed) and get 512Mb Corsair XMS3200C2. Then you should really be able to fly.
With my setup above I have two fans in the case (+ one on the CPU and one in the PSU). This appears to be adequate for what I am doing now, but I may stick a fan on the Northbridge heatsink as well.
Thanks, guys!