These processors are practically identical, as they both utilize the same core revision. ( Codenamed "Italy" ) The only difference is that the Opteron 280 operates at 2.4GHz and the 275 at 2.2GHz. The "Italy" silicon revision includes dual processor cores, 2x1mb of L2 cache (1mb per core), and is produced with the 90nm manufacturing process.
AMD vs Intel:
Unfortunately, determining a comparable Intel processor depends on the general roll of the server. There are numerous reviews, benchmarks, and comparisons available online which will aide in answering this question. The following link is a comparison between the AMD Opteron 275, Sunís UltraSparc T1 1 GHz /w 8 cores, Intelís Xeon "Irwindale" core at 3.6 GHz, and the Xeon "Woodcrest" core at 3 GHz.