They look differently: DDR2 and DDR. You can't put them in, period. If you did, you've ruined either the memory or the motherboard, or both.
For such old computer, the video card can't be very good. Maybe for normal browsing. Sure it might have DVI, but may not support blu-ray or anything. No need to keep for long.
DDR 400 memory is not cheap these days, and the max you can find is 1GB.