A slow system can't be blamed on "slow" memory entirely. DDR2 is fairly fast in it's native state. Value ram isn't the best, or even average memory by any means though. It's enough to run a system and not much more.
First, PC4200 states the standard in which catagory memory falls into. PC3200 is DDR400, which runs on a double-pumped 200Mhz bus (400Mhz).
PC4200 is DDR533 is memory rated at 266Mhz. The PC rating is a rough estimate of it's proposed bandwidth it "should" offer. PC3200 should be able to allow 3.2Gb/s.
The ratings are pretty much the same on DDR2, but it'll be PC2-4200 instead of PC-4200. It's proposed to offer 4.2Gb/s in theory, but may exceed that right off the bat, or dive under that. So, it's really just a clock speed rating these days. Why? A chipset could be holding the bandwidth back, or the memory itself just may not allow the bandwidth even though it can hold the clocks. It's only in classes above value chipsets and value memory modules that you can actually count on the bandwidth reaching or exceeding it's rating.
Good DDR2 is going to cost you through the nose. That value stuff can get ya a Gig for $134, While I had to fork over $350+ for a Gig of premium DDR2. The best DDR2 is still OCZ DDR24200 Rev2 Platinum, and is running at $305 for a Gig of their Dual Channel set.
Oh! You may be totally overlooking Dual Channel capabilities and is something every user should set as the basic choice first, then the rest can be weighed. If you want better memory, look outside of the value line and only in DC kits. But I have a strong suspision that it's not your memory that's making you feel slow.
So why is your PC slow? What do you have in there as the base of the system (CPU/Motherboard)? There's a LOT that can make your PC seem slow, and it may just take good parts to get it up to steam. Like, you could have some P4EE on a 925xe chipset, only to have a slow HD, like an old 4m cached 5400RPM, drive drag it all down and it feels slugish. One stick of 512Mb is not running dual channel, and hardly offers you any capacity for intensive apps. More than likely, it's running out of physical memory and going into your slower hard drive to use the Virtual Memory.
And DDR2 isn't going to be stocked locally for a while. You're lucky if you can even find good DDR in local stores. I think it was just this past year that I have seen Corsair XMS DDR hanging in the isles at Best Buy. Mostly they all carry budget stuff like Crucial, PNY, etc... It's all just enough, and not much more. Now factor in how many of us actually have DDR2 systems, where you buy it, etc...then you get into overhead costs of just ordering and stocking this stuff by the store's manager. It could sit there all year and not get bought. It has to be more widespread before they'll stock it. Same with PCIe video cards. Besides the X600/700, you can't find a PCIe card on a local shelf. That should change soon because both platforms now offer it up, and the craze is huge. But DDR2 has no craze...
Need more info...