Pavilion a210n Memory Specifications
Standard Memory 256 MB (removable)
Maximum Memory 1.0 GB
Memory Expansion 2 sockets
Memory Comments PC2100 DDR SDRAM DIMMs
CPU Type 2.5GHz Intel Celeron
Model Comments 400MHz FSB, Intel 845GL Chip
Most information that's available for Socket 478 PCs is from when those motherboards were manufactured and when the largest RAM modules were 512MB, so 2 sockets summed up only 1GB... but the largest DDR modules made are at least 2GB and possibly up to 4GB (found no info yet to verify 4GB sticks were ever made)
I own a Dell Dimension 4500 with the D845 Chipset that according to the Corsair RAM configurator, can use 2GB and according to Astra32 (System Information application) the total capacity is 3GB (3072MB).. I've also read some users that claimed to be running 4GB on a Socket 478 computer, so it's more than likely your computer can handle at least 2GB of RAM, and more than that would have to be analized by a RAM configurator considering their information is most likely outdated, or with a System Information Application.
Some comments or the largest manufactured DDR RAM modules
http://www.tomshardware.com/forum/226263-30-largest-dimm-market
1 & 2GB DDR RAM for Sale.
http://www.ebay.com/sch/Memory-RAM-/170083/i.html?_dcat=170083&Type=DDR1%2520SDRAM
Windows 7 system requirements
http://windows.microsoft.com/systemrequirements
Microsoft Office 2007 System Requirements
http://www.microsoft.com/officebusiness/products/technical-requirements.aspx