i work with absurdly large files (up to several gigs). add a few layers, then even 32 gigs is not that much.
i try to configure a single cpu-system. A single quad-core is sufficient for photoshop i think. 8 cores instead of 4 add negliglible performance. maybe some filters can really use 8 cores, i´m not sure about this.
switching from a 4gigs to a 32gigs system removes the ram-bottleneck. so some opterations may only take seconds instead of minutes. if an operation takes now 3 or 5 seconds is not so important. i do not want to invest shedloads of money for a few percent performance advantage.
Current intel cpu's have better performance figures in photoshop as compared to their AMD counterparts.. However, you wont be needing a xeon or an opteron for that.. A single i7 930 put together with 12 gigs of DDR3 ram and a mid range video card (preferably nvidia) with high memory (1GB or more) will suffice.. If your budget permits you, get a cheap SSD which will definitely boost up performance as compared to conventional hard drives..