Sorted my problem.
It's something that I've encountered before. The motherboard in my system is an ASRock AM2NF3-VSTA, with an nForce3 chipset. I did have a couple of Seagate SATA drives in there. I have now swapped them for two Western Digital WD2500AAKS 250GB drives - and the performance has gone up markedly.
With the Seagate drives, I was getting around 2Mbs and 8Mbs copying data to them over a Gig-e network. With the WD drives, I'm now getting around 17Mbs!! A vast improvement. I managed to copy over 60Gb of VMs in a few hours, no problem. And that was via the VI client, which previously ran like a stunned snail!
So I can confirm, the following spec appears to work perfectly fine with ESXi 3.5.0:
ASRock AM2NF3-VSTA motherboard
AMD Athlon 64 4000+
4GB Crucial RAM
2x Western Digital WD2500AAKS 250GB SATA hard drives
Liteon DVDRW IDE drive
GeForce 4MX AGP graphics card
Intel Pro1000MT NIC
3Com 3c905TX-CMX 100Mbs NIC
We're not using the onboard NIC as I disabled it some time ago for other reasons, and just went with the cards already in the box.
Now I just need a little iSCSI box, and I'm laughing. FreeNAS here I come!!!