I skimmed through all this, so if anything I say is a repeat, sorry, I'm lazy.
First off, LFS is all about them mhz. Video card memory has nothing to do with it, unless you are going from 32mb to about 128mb. Anything above 128mb on AGP is almost useless, I think the AGP bus can't use any more than 132ish mb. As for the PCI bus, It's something like 50mb. PCI-E is the future, as it's twice as fast as AGP, so it can use all of the 256mb and some. If you are only playing LFS, make sure you get a card with a high GPU speed, as that will make more of a difference than memory. The speed of the AGP slot is also important, when I had my old gateway, it only had a 2x slot. Came with a TNT2 32mb stock, but I took my Ti4200 128mb and set it up just for kicks... The 4200 actually did worse then the TNT2.
To who ever started this thread, the only way I can see you getting double the frames is if the FX5600 was on the PCI bus, and the FX5500 was on AGP. And since it's a dell, that is very well possible.
And last but not least, for my FX5700LE 256mb, my 9800pro blew up and that was the best/cheapest/best performance replacement I could find. Got it for $40 canadian
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****I would never buy this card, or any other FX series card new.****