Older games (eg, pc games that wont exceed half your limit to download) can be difficult to find active download links/torrents for in all honesty, and *some* of the time if you're successful with getting it, it won't work, or will require a patch that's hard to find or no longer available.
Good thing is though, ebay is real cheap for "old" pc games.
No money at all? :
As for ps1 emulation, some guys here probably know something about it...although I've been told n64 emulation is all good and encountered nothing but poor performance.
So, I just go with arcade stuff, 8-bit, or 16-bit emulation.
Small downloads, way better performance, LOTS of it around.
Check out
http://www.get-your-rom.com/ for lots of those.
or if you wanna see the games you can get, here:
http://www.get-your-rom.com/roms.php
Lots of stuff there, the huge majority has been in great working order.
Haven't yet figured out Sega cd though...