don't worry about the P5B QVL
I disagree with this advice. You still need to pay attention to the QVLs. You do not need to buy listed RAM because there are simply too many RAM makers and models for board makers to test and list them all. But you do need to buy RAM with the same specs as those listed on the QVL.
They can test all the CPUs so the CPU QVLs are typically complete, and you should buy a CPU that is listed.
Oh, and there is a reason why that QVL is years old, so is the motherboard and that motherboard does not support anything larger than 2Gb sticks. And the supported 2Gb sticks have been around for years too.
Technically, there is no reason why you cannot buy a single 2Gb stick and add that to your 2 x 512Mb. The 2 x512Mb will run in Dual-Channel and the 2Gb will run in single channel mode. But you would get better performance if you bought 2 x 2Gb and installed them in place of the 512Mb modules. Then, if you had 64-bit Windows, you could move the 2 x 512Mb to the second bank (channel) of slots (B1 and B2 on your board - according to the manual). But you have 32-bit so you might just as well toss the 512Mb modules and chalk the loss up due to advances in technology rendering perfectly good RAM obsolete.
Although it is very difficult to find RAM that is not warrantied for life, I also recommend sticking with a major brand. And most major RAM makers have RAM wizards to tell you which of their RAM will work with your board. Here's my canned text on that.
These popular RAM makers have auto-scanning and/or manual entry RAM wizards to help you determine which RAM is compatible with your motherboard. For manual entry, enter/select the PC or motherboard make and model number and the wizard will list compatible RAM.
Crucial - Memory Advisor
Link Removed due to 404 Error - Memory Finder (manual data entry only)
GeIL - Memory Meter
Kingston - Memory Search (manual data entry only)
Mushkin - Advisor
Link Removed - Invalid URL - Memory Search (manual data entry only)
Link Removed due to 404 Error - Memory Configurator (manual data entry only)
Link Removed due to 404 Error - Memory Finder (manual data entry only)
The following retailers have auto-scanners and manual wizards. They sell brand name and/or "house" brand (re-branded) RAM.
Newegg - Memory Configurator System Tool
MemoryStock - Upgrade Configurator
18004Memory – Configurator
4AllMemory - Memory advisor (automatic and manual)
Link Removed - Invalid URL - Memory Configurator (manual data entry only)