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My KDE4 mem when booting to KDE 4.7.3 is 4xx MB. I'm afraid it'll not enough if it just 512MB mem (unless you kill akonadi & nepomuk). Barely usable. It'll run slowly cause then have to use swap space.
When you just have 512MB mem, when booting to KDE, Try to you open firefox with 4-6 tab, a couple of program and you'll run out of mem.
I really like KDE, but their mem footprint is worse with each release. Memory's cheaper now doesn't meant KDE have to throw all the memory they can use.