As long as you have matched memory pairs on each channel, you should be able to run dual channel.
The only potential problem: Because you are using 4 memory modules, which require more power, your system may clock the speed of the modules down to maintain stability.
2x512MB is the best setup for WINXP.
Only certain applications and games will benefit from more than 1GB. Games like World of Warcraft, Farcry.. etc.
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"Memory with lifetime warranty? So, whose lifetime is that?"
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<font color=red>AIM BrentUnitedMem<font color=white>