In the chart below you will find the processing time for a number of filters in Adobe CS4 using similar systems, just with different CPUs. I used to be an AMD fanboy but AMD has really fallen behind when it comes to CPU performance, the Intel i5 750 beats any quad core AMD, and any faster Intel chip does as well...but it's worth to check the pricing as well, there might be certain pricepoints where it's still better to buy an AMD chip.
Charts, benchmarks 2009 Desktop CPU Charts (Update 1), Adobe Photoshop CS 4
But as mentioned earlier, both Intel and AMD have released 6-core chips lately, and while the Intel chips are much faster, they are also more a lot more expensive, so only AMDs 6-core is really worth it, but then again it barely beats the Intel i5 750 in photo processing in CS4 (look at the second link), which is almost one ghz slower and only has four cores...but other than in CS4, the 6-core AMD is faster than that particular Intel chip most of the time, so again, the price will have to decide which one to go for.
Benchmark Results: Productivity : AMD Phenom II X6 1090T And 890FX Platform Review: Hello, Leo