Your processer is majorly bottlenecking that card. Its a very CPU dependent card, when I got it having a 1.2 ghz Athlon something, my old Geforce FX 5200 was much better. I upgraded to an Athlon XP 2800+, 512MB of RAM, etc.
But I really see no way how you could get all the benifits on a celeron. Big question though, what happens in other games? HL sometimes has problems with the big ATI cards, its usually fixable though. But if the performance is really bad in other games to, then I'd say CPU bottleneck.