Ah nice, that's almost my old PC configuration, but as I remember it often struggled even with FH1. The main problem is the CPU, 1.8 GHz just doesn't cut it anymore, especially with bots. Also, I think more RAM would be a nice idea, you can get this from a friend I'm sure, people usually have lots of old RAM sticks lying around, the question is only if you motherboard has empty slots for them?.
Unfortunately the Athlon 3000+ is single core, so even trying out something like HABAI:
http://www.moddb.com/mods/habai...that improves framerates with multi-core CPUs is not going to work.
Your best bet is a slight overclock of the CPU, I remember mine could be somewhat overclocked, but make sure your cooling is adequate and the cooler is clean from the dust buildup (new thermal paste wouldn't hurt either). Otherwise try to find someone who's throwing away his 10 year old PC with a dual core socket 939 CPU or even a newer, faster socket 939 from around 2006 for a performance boost. Here's the list of AMD cpus
http://en.wikipedia.org/wiki/List_of_AMD_Athlon_64_microprocessorsAnything from the 4000 and above series is probably a good upgrade if you can get it used for around 10 €.