That more expensive memory is that way because it's getting somewhat harder to find. That's not to say that it's a good choice. It's definitely NOT what you are looking for for your system, as Gambino the Great said.
You'll want at
least PC3200, and at
least 1 GB of the stuff. 2 GB would be better, and would probably be the best upgrade after a year or so of owning that system. You're just a small fraction away from top of the line in terms of desktop computing power (without any fancy cooling or overclocking). Though more RAM will only allow you to run future games more smoothly, and any other things that involve manipulating large amounts of data.
The other thing to note is that with the 939-pin socket, you can swap it out in the future for a 939 dual-core chip (Athlon64 X2). But the first round of dual-core chips won't offer too much in the way of a performance increase because they are the first stab at the engineering problems associated with those chips. AMD's internal quality assurance testing is good, but they can't catch
all of the possible CPU+hardware combinations and software uses out there. Plus, you're better off waiting a few months for the price to drop for the dual-core chips when you do your future-upgrade.
The other issue to remember is that you can get a version of Windows XP that is made for 64 bit computing. The only problem is that hardware manufacturers are still behind the curve for writing new drivers that work in the 64 bit environment that you have with WinXP 64 and an Athlon 64 chip. To fully utilize your computing power, you'll need to run linux.
The plus side is that you're on the cutting edge of the transition to 64-bit stuff and it's only a matter of time until programs are modified and re-compiled to make full use of the 64 bit environment.
As far as the PC 3200, PC 3500, PC 4000, etc...
The designation used to be related to how much data could be pushed across the memory bus in one second (PC 1600, PC2100 were 1.6 Gb/s and 2.1 Gb/s, respectively). The bus data rate is related to the clock speed of the bus, so if you increased the clock speed of the bus (and used Double-Data-Rate memory chips), you would have a higher amount of data that could be pushed around. Nowadays, PC3200 effectively operates at 200 MHz bus speed, but it's almost the exact same chips used a few years ago. (They're refined versions of the old designs, so they can handle the faster switching of the voltages on the bus.) The bus is running at 200 MHz, but the chips are DDR, which basically means you get twice the rate of the bus (400 MHz). Which gives you:
8 bits * 400 MHz = 3200 Mbits / second or 3.2 Gbits / s.
for PC 3500, the bandwidth should be 3.5 Gbits / s. (A bus clock of 437.5 MHz / 2 (for DDR) = 215.25 MHz)
for PC 4000, the bandwidth should be 4 Gbits / s. (A bus clock of 500 MHz / 2 (for DDR) = 250 MHz)
The bus clock is what frequency your CPU is being pumped at, and I believe the PCI Express bus is pumped very close to that. On some motherboards you can only change two (memory/system bus, and PCI Express). On others it is three (PCI Express, memory, and system bus independent of memory).
In general, go for good quality and price of the kind of ram your motherboard can work with (PC3200 in your case). If you are going to do any overclocking, you'll want RAM that is capable of running faster (PC 4000). But always try and get dual channel ram; it will help make your next upgrade of more ram that much farther out in the future.
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