first, do free checks.
1. see if there's a bios update for your motherboard
2. pull out 128mb dimm and see how it works. if there's no change, pull the 256 one, but leave teh 128 in. troubleshoot. if it shows a different number, something's wrong with the ram identifier on the dimm chipset
3. the above comment about a better videocard is dead on. PM me and remind me to look around, I may have a spare diamond 32 meg agp somewhere, I think. You can have it for cost of shipping if I find it
4. Lose the duron 700. That's about as fast as a celeron 400. It has 64k cache, and is DOG slow. Find a used athlon xp 1500+ or higher (number does not equal actual speed, 1500 is more like 1.2 ghz) and use that. 384k l2 cache > 64k l2 cache
5. I'm not sure if that motherboard properly allocates ddr memory. I would try pc133 if your board supports it, you may ahve better luck. Something tells me you have an ECS K7S5A motherboard that supports both PC133 and DDR memory banks, but only one set at a time.
to those bashing XP: it's actually one the most robust OS microsoft has produced for the end user since windows 3.11 (which wasn't even an OS). Mine hasn't BSOD'd since I've installed it 2 years ago. I had a small problem with first release of SP2, but system recovery actually did a wonderful job on the rollback.