i'm glad we could help.
don't buy eReader format books for your eb1150 ; it can ONLY read .imp format natively. if you want to buy books which are not available in .imp format (from fictionwise, etc.), i recommend buying LIT (Microsoft Reader) format (this format is also available from fictionwise : they promote the eReader format because they own it), since this format can very easily be converted and is basically a very thin wrapper format around an oeb / html core.
for non-drm books the mobipocket format can also very easily be converted (i can't remember now whether it's possible de de-drm mobi books, so just to be safe you can stick to lit). i recommend our very own nrapallo's tools for conversion, namely lit2imp and mobi2imp (take a look around the forum here to find plenty of information and the tools themselves).
as for the selection on filament books, to be honest i've never bought anything there, i only use their service to transfer word files over (before i learned how to make my own imp books, or as i said when i'm feeling lazy). i don't ever buy an .imp format book because it is a proprietary "dead-end" format (with some other serious limitations), meaning it can only be read using the eb1150 software and cannot be (easily) converted to any other format. if one day my eb1150 dies and i cannot replace it, or i switch to a different device, i want to be able to read my books on a different device. so i prefer formats which i can easily convert, or archival formats like html (if it's a project gutenberg book).