Is there a system limit to memory size?

Hello,
My pc has two memory slots each holding a 512 MB DDR. [in fact, a recent enquiry to your website enabled me to get the use of both slots - thank you.] the machine runs quite well for my purposes, but im interested to know if i could upgrade one or both cards to 1 GB, to get a total memory of 1.5 or 2GB. Or do you think there is some system limit imposed by the motherboard, which would prevent me getting any mileage from bigger memory?
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my machine is a viglen omnino about 5 years old, model number L297T.
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i ran cpuz, which tells me that the motherboard is CLEVO model L29xT, and CPU is intel pentium 4630 with speed 3GHz.
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the memory is 1024 MB DDR dual channel. Each 512 MB DDR is made by micron technology, part number mt8vddt6464ag-40bd1, max bandwidth PC3200 (200MHz).
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i have to confess i dont know what all of these details mean, but ive tried to quote the details that seem most relevant (!) My question is.... If i took the memory DDRs into a good store, would i be able to upgrade one or both of the DDRs to 1GB, to get a total memory of 1.5 or 2GB? Do 1GB DDRs exist, compatible with the DDRs i currently have? And is there a system limit to memory size which would prevent me getting any mileage from bigger memory?
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i'd be grateful for any easy-to-follow advice.
yours askingly
tim