If you're talking about Socket 754 CPUs, it depends on your needs and expectations. For most applications, you'll hardly ever notice the difference between 2800+ and 3000+. Personally, I'd spend the 2k for something else such as a better gfx card, more RAM, speakers, etc. These things will produce a much more noticeable difference in performance. However, if your budget allows you to buy all these other things AND a 3000+, then go for it.
OTOH, 3000+ is the lowest processor available in socket 939. The advantage of S939 is that motherboards with more future-oriented features are available for them, such as PCI-E slots.
I use the term "future-oriented" instead of "future-proof" because nothing is really future-proof in computers except for a short period of time.
I would strictly advise you to look into the aforementioned processor's reviews (here's one from xibitlabs.com) and to decide for yourselves if the perf diff u see are justifiable, relevant to the type of app u will be running.