SHORT ANSWER:
Intel Core 2 Duo E6750 system with 2GB-4GB of RAM, and 400 GB to 500 GB disk.
Run CentOS 4 x86_64 (aka AMD64) 64-bit OS
https://www.centos.org
MUCH LONGER ANSWER:
I have worked at Synopsys for most of the last six years and had the "Platform Marketing" position for about six months - mediating between the development groups (who only would like to support ONE platform if possible), the customers (who want every possible combination) and the OS vendors (who always claim the customers absolutely must have some particular combination).
See this site for the officially supported compute platforms for any given release opportunity period:
https://www.synopsys.com/products/sw_platform.html
Having said that, I can also tell you from personal experience that CentOS makes a perfectly viable alternative to Red Hat Enterprise Linux.
I personally recommand a dual-core x86_64 system, AMD 64 or Intel EM64T, with the x86_64 64-bit version of CentOS - currently I'd recommend CentOS 4 == RHEL v4
Your most bang-for-the-buck right this minute is probably an Intel Core 2 Duo E6750 processor based system, but note that what is the best deal changes almost monthly.
Get at least 2GB of RAM, 4GB recommended, and a fast several hundred GB disk - say 400 GB to 500 GB.
Warning, in the past cheaper motherboards have not supported Serial ATA (SATA) very well but a newer mainstream motherboard should be fine.
Back in 2005 I built an Athlon 64 2800+ single core, 2 GB RAM, 300 GB Parallel ATA (because the SATA support is flaky on this board), on a Gigabyte GA-K8U. This is an obsolete socket 754 system and I think the slowest AMD64 they ever sold but it runs the Synopsys MAP-in* program software just fine.
This system is running CentOS 4.5 (RHEL v4 update 5) x86_64 with 2.6.9-55 kernel.
*MAP-in provides access to the Milkyway digital design database to other EDA vendors for free. See **broken link removed** for more information.
LCBrevard
San Jose, CA
**broken link removed**