I am trying to help people......so please take that into account when wording a reply.

(The wording you used makes me out to look like a guy who tried to sell someone swampland or the Brooklyn Bridge.)

Also read the printout. It says to use registered ecc memory near the bottom.

Helping people by telling them to buy memory that will not work in their computer is a poor form of help.

In fairness, I can see how Sandra's "performance enhancing tips" would be confusing. But they are not valid for anything but very large servers and have no relevance to the motherboard configuration or specifications.

The existing RAM is this: Corsair CMX512-2700C2 512MB DDR PC1-2700U DDR1-334 (2.5-3-3-7 2-10-0-0)

Non-registered, non-ECC. Do not mix RAM types.

I made a mistake. No need to chew me out so much over it. Usually I put more research into the matter and for that I apologise to OP.

An aside...one site listing that mobo says RAM should be at least 1.5GB....wtf is up with that? Does that mean for single channel optimal operation?(3X512MB) with one stick in each slot at all times? Cuz if so OP having only two slots filled could be slowing him down a bit as well.

RAM configuration on those 3-slot AMD Socket 754 motherboards is screwy.

You can go up to 3x512MB, or 2x1GB. You can mix 512MB and 1GB sticks. You can't use 2GB sticks or exceed 2GB. There may be individual exceptions, but there's nothing indicating that this motherboard is an exception.

There's no interleave, so you don't get benefit from filling all or even two of the slots. Memory interleave wasn't introduced until Socket 939 and later CPUs with dual-channel memory controllers.

Best bet for the OP: Get one 1GB stick (DDR-333, CAS 2.5; anything faster would be wasted). Now you have 2GB, which is the maximum for that motherboard.

This says 3GB max, and so did someone else in this thread as well. Odd.

hmm

Claims by memory retailers, who may do no more than multiply the number of slots by the largest permitted size, are not authoritative.

Any Socket 754 motherboard, particularly those with VIA chipsets, should be considered 2GB max unless otherwise stated by the manufacturer. Since the manufacturer states 2GB for this motherboard, that trumps any claim to the contrary.

Just for the records.

There were several generations of Athlon 64.

The very first one, codename Clawhammer, 2 Ghz, 130 nm, socket 754,FSB 400 was released on 23 September 2003. These were the very first AMD 64 bit processors. And they were quite a match for their Intel counterparts.

There were quite a few other models, the best knwon being the Manchester generation ( socket 939 ) and Orleans generation ( socket AM2) in 2005 & 2006. They had better dies, better FSB and lower TDP.

the op seems to have a venerable clawhammer.

Hence the interest to clean the dust and change the thermal paste. Clawhammers had a 90W TDP, quite considerable at the time.
Same gerenation, so should work, but as someone else said before, be careful that this will cancel any OEM software used on your computer.

If you go that way, note that the best you can expect is a 10-15% increase in performance

The manual I have said it supports 3GB
Quote “The K8X800 Pro II provide 3 sockets using 184 pin DDR SDRAM. You can install DDR400/333/266/200 (PC3200/ 2700/ 2100/ 1600) SDRAM. It supports a total capacity of up to 3GB with DDR200”

Maybe it means you can install 3GB of 200 speed but go higher and you can only install 2GB?

A new processor will cancel my OEM software?

New CPU or mobo sometimes invalidates your MS OS copy as the OS when installing copies some info from both parts to prevent piracy/etc and checks certain hardware upon bootup to see if it's the same.

If you're upgrading in a few months, you can probably just hold out until then. Why mess with the machine? You aren't having any issues with it and it will continue to run older games just fine. Upgrading memory seems unlikely to provide a significant performance boost considering the system as a whole. If you do decide to upgrade the memory, be gentle. A motherboard of that age is delicate, old electrolytic capacitors may be on the verge of failure.

Why would a new CPU invalidate an OEM copy of Windows?

All I know is it can happen....the OS records info on your system upon installation to prevent you from moving that OS copy to another system via USB drive/DVD/etc, as it checks certain hardware against it's lists upon bootup.

Never had a processor upgrade invalidate an OEM copy in my experience.