The SS51G is a nice box, but do NOT waste your money on a P4-based celeron of any speed. Even a P4 1.6A is faster than any Celeron of any speed on the market right now, because of the difference in cache size and the P4's natural sensitivity to memory bandwidth. Benchmarks of real-world apps show that it takes a 3GHz Celeron to perform as well as a 1.6GHz P4 with 512k cache. With the P4, the cache makes that much difference.
That said, the cheapest genuine Northwood (512k cache) P4 you can get your hands on will be ample to do the job you're looking for. A 1.8A chip should be plenty for your needs, and retails for $129 on NewEgg. If that's too much... consider going for an AMD solution (Nforce2-based SS41G2, if you can find it) instead, since you can get an Athlon XP2500+ for $96, and the Celeron is simply not worth your money.
Edit: To answer your original question, the SV25 with a high-end Tualatin should technically be enough, but you can get a much better box for a little more money, either Athlon or P4-based, these days.