yes, 512 is the official minimum but you won't be able to do much at decent speed with that ammount of ram with kde4, usually you will need 768 or 1 gb to work normally
now, the i810 is the chipset or the video card?
you mention card, do you use the onboard video card or a pci or agp card?
some old mainboard with intel chipsets required to disable the onboard card, verify bios
others allowed to set the amount of ram taken for onboard video card, verify bios too
old intel cards can take a portion of each ram module depending on the size, sometimes is better to leave only one 512 mbs ram moduel than two 256 mbs modules because bios will take less form each module than form one module, the other problem with i810 video cards is that it takes weirds amounts of ram, for example 5 mbs or 10 mbs, usually you need 16, 32, 64 or more, especially for kde4 effects you will need at least 64 and i810 won't take that mcuh, i beleive that the limit for it is 24 mbs