Well, for starters, that Barton 2500+ is almost definetely multiplier locked.
That 2600+ looks nice, seeing as how it is unlocked, is a Barton, and is probably the second cheapest of the three.
And I'll bet that the 2800+ will cost you the most out of the three.
Do you know the *rest* of the stepping for each chip? More info is always nice
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Since you have an ASUS A7N8X Deluxe rev. 1.04 Motherboard, with PC 2700 RAM, you want to really do want to skip the locked Barton 2500+, simply because you will have trouble getting a 200+ FSB with that motherboard, and RAM.
The 2600+ is probably your best bet of the three - again dependat on price.
And the 2800+ is probably being sold for more than it is worth to you as an overclocking chip, but otherwise would be a decent choice. I would avoid it simply because higher rates chips are a waste of money - seeing as how the lower rated chips perform just as well.
If I were you I guess I would just skip em' all, and look for a cheap Thoroughbred "B" 1700+/1800+. But if the price is right, than you may as well jump
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