upgrade man, if your ram can only operate at 100...im guessing your motherboard and ram have to operate at same speed 1) Get more ram, at least 2 512mb sticks to make 1 ghz if your computer can operate with 512. Then get a maore compatable graphics card. I think you could settle fro a Geforce XFX 5200 Ultra. Not sure if your computer could handle it but i think that would be the cheapest and best performance increase for your computer to run HL2, hope i helped
You can deal with that video card.
I'd suggest getting more RAM
512MB/64Mx64(32Mx8/16C) Generic SDRAM-PC133-PC100(168-Pin DIMM) http://www.arismo.com/customer/product.php?productid=19698&cat=594&page=1
Each 512stick is $35, so get as much as your MoBo can handle.
I'd suggest getting a new processor....what kind of MoBo do you have?