Different speeds of RAM does not allow you to expand even more. If you're motherboard is limited to 2GB, then you can only use 2GB. Wheither it's PC2100, PC2700, or PC3200, it makes no difference.
Download and run CPUz and see what speed RAM the system's use. I suspect PC3200. You can use slower RAM, but you'll obviously loose some performance. Same goes, if you upgrade to PC3200 but he mobo supports PC2700, you'll be limited to 2700.
Even if these systems could support 4GB, that's overkill for even some of today's current systems. 2GB is more than I'd even stick in those machines. 1GB should be plenty for each. You have to think of the hardware. Just because they have more memory doesn't mean they will work faster. You're still going to be limited by the processor and what not.
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Urban De-Development
* Athlon X2 4200 | 2048M RAM | 160G HD | 7600GT
* C2D T5250 | 2048M RAM | 160G HD | Intel x3100
* G4 1.33GHz | 512M RAM | 40G HD | Radeon 9200
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