AMD is the flavour of the month. They tend to score better than Intel's in some bench marks but sometimes it's the other way round. It's upto you, you'll notice almost no real difference in performance at that level.
I would think the Sempron 2500 would be much faster than the Celeron 1.4. If its value for money you're looking for go with AMD and the Sempron. Good advice from Diodorus Siculus though, both these PCs will be based on totally different motherboards as they plug into different sockets, so check they both offer the same upgradability such as the same number of PCI slots, USB ports and have at least an AGP port for graphics card upgrades.
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