Beetlejuice - All the memory modules suggested, will fit your rig. You motherboard configured for the PC3200 / 400 MHz DIMM 184-pin, so that is the max you will be able to run on it. You can actually plug in the 533Mhz 184pin into it, but due to the Memory BUS limitations of your motherboard, it will downscale it and only run at 400Mhz. The only time it will make sence to but the 533Mhz module will be if you will find it on sale or clearance for less than the price of the 400Mhz.
However, in reality, to the average user, the 400Mhz is more than enough. I have a few systems at home, and when running BF1942, UNREAL, Diablo II, and other games, I can't tell the difference in performance.
It all boils down to the video card you have. If you have a decent card, you don't need the fastest memory. Cards are somewhat over rated as well. I am still using the ATI Radeon 9800XT 256MB, and it pulls anything I throw at it even if this card, according to the current trend on the Video Card market, is considered to be a grand grand father..
Now, to your system - usually, that HP model will have the ASUS K8S-LA (Salmon) Motherboard with a Socket 764 AMD Athlon 64 processor. It only has 2 memory slots ( you can see if I am correct by opening the case and peeking inside ). The max memory the MOBO will support is 2MB, 1MB DIMM in each slot. That model usually ships with a total of 512 ( 2 x 256 DIMMs ), and if you decide to upgrade it, you will see a significant boost in performance. My suggestion to you is to go with the 2 x 1 MB, for the maximum supported. Although 1 MB is very common in the computer world, it is still a basic size. 2MB will allow you to run games smoother while other applications are still loaded in the background ( Anti Virus, system updates, windows applications, etc ).
