Yeah, you are in a tough spot. I have two similar systems on my bench now. A KTE Duron 900 and a KT133a TBird 1.2GHz. Both need new HDD's, but besides that I hate to advise them to sink more money into those systems as they are well balanced now with the components they have. The D900 isn't a gamer, but the TB1.2 is used for games.
Anyway, back to you. A video card will help you alot in many games. But the system still won't be up to specs for HL2. I'd OC it and look for a cheap used R8500 or better yet GF4Ti. Otherwise, you could upgrade that KT133A mobo to an Athlon XP I believe(?), but it's still a KT133a with SDR mem. Hard to know what to do in that case, but a cheap card and OC will definately help in many cases. But Farcry, HL2, D3, MOHPA... forget it.
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