1500 KB/s is pretty unacceptable for a retro rig, especially a 1 GHz unit. The last time I net benchedmy dual 850 MHz box, I got above 9 MB/s using a stop watch. This was in XP Pro.
Even my 486 super server gets 2500-3000 KB/s in Windows NT 4.0 using a 3Com 3c905C-TX-M 10/100Base-TX PCI card.
If I were you, I would isolate the problem and test even the less obvious ideas. Here are some suggestions:
1) If you are on a dual-boot machine, test the network through-put in NT/W2k/XP.
2) On slower machines, I have noticed up to a 50% speed difference depending on which of the two computer's initiates transfer. For example, looking at a 486-to-1GHz transfer, the transfer will go much quicker when the 1GHz box initialises transfer compared to the 486. Have you tried having the XP box initialise transfer?
3) Check for shared IRQ's with the network card. Non-NT Windows operating systems do not do a very good job with IRQ sharing. If there is sharing, try changing the IRQ order in the BIOS to change the IRQs.
4) Throw in a different style network card, such as an Intel, Linksys, or Netgear PCI network card.
5) Another user mentioned cable shielding. I always just assumed that the NIC side grounded the shield. If both ends are, for some bad reason, grounded and you have a long-ish cable, then you will have a nasty ground loop. You should check for this with a multi-meter. Very very funny things can happen in ground loop situations. If you do not have a shielded cable, use as short of a cable as possible to minimise the question of interference.
6) If you are still using the Linksys firmware on a Linksys router, try a full reset of the router, not just a reboot reset. You'll need to re-enter all your settings, but I have noticed that Linksys routers just start acting wonky after some time. I recommend switching to OpenWRT or Tomato for your router's firmware.
7) Ensure that you don't have any ports forwarded on the router which are the same for more than one IP address. Funny things will happen in this case. Also ensure that you aren't trying to forward a reserved port.
8) The 3Com cards have a Windows GUI utility which lets you change various options like network booting, IRQ's, addresses, etc. Have you played with this?
9) Try using Windows Explorer to transfer files. If that works fine, then you have an HTTP and/or Firefox issue.
Plan your life wisely, you'll be dead before you know it.