What is the minimum configuration of computers for best performance?
As for minimum configuration -- it's recommended to have one CPU core per each GPU core though fast enough CPU usually can handle several GPU threads. It's always better to have, for example, 2.4Ghz Quad Core rather than 3.0Ghz Dual Core. Additionally, final password validation stage for WinZip/AES is -- hashing the whole file, this stage done on CPU only and so if you're trying to attack very large file then CPU can be a bottleneck (for OpenOffice CPU is always a bottleneck and for MS Office documents CPU isn't that important).
All in all, to build 2xATI 5970 system it's good to have:
1. some stable MB with at least 2xPCI-E slots (Asus producing many suitable MBs even with 3-4 slots);
2. Quad Core CPU (Intel highly recommended, HT is good but not that needed, so old Core2 Quad or i5 are OK too);
3. 2-4Gb RAM (amount of RAM isn't that important for password recovery, just minimum amount to run Windows smoothly);
4. 800W+ PSU from quality vendor;
5. properly cooled case which can accept 32cm long video card, additional fans are very welcome and water cooling recommended.
To make all GPUs recognized by system you'll also need to attach monitor/dummy plug to each video card. Running in completely headless mode can take some additional actions but I guess more dummy plugs can handle this.