let me guess - bout 5fps on lowest settings!
just upgrade your graphics card - if moneys an issue - you can pick up a budget card that will still do the trick -
Radeon 9200 SE
Sapphire 9550
etc. these cards are bout ?40 retail - nothing stupid - and will definately allow you to play HL2 stable.
However, if your maps are likely to be a LOT less complex - then it could be OK, its just down to how long it takes
your maps to load - hopefully far shorter. Otherwise, no, its not worth mapping for HL2, you will kill yourself - end of story!