I had the exact same requirement as you i.e to learn Oracle RAC with cheap commodity hardware in a home environment. I went with an AMD 780G based mobo with a 2.6Ghz AMD Dual Core. I packed with 8GB DDR2-800 which costs less than $100 nowadays. 1TB and 1.5TB disks are also reasonably cheap from newegg. Use Linux software RAID and LVM to speed up the disks..

I chose the recently free'd Citrix XenServer and I am mightily impressed. It just wipes VMware off the floor when it comes to I/O performance due to para-virtualized drivers. The NIC drivers are also fast and efficient. I solved the shared disk problem by making one VM an iSCSI target. I can present as many LUN's as I want to other VM's now. since all traffic goes over the internal NIC, performance is excellent.

The XenServer product also consumes far fewer system resources than VMWare and the quality and polish is really good. not to mention the excellent documentation that is available from Citrix

You can save a lot of money by choosing AMD based chipsets and a cheap Phenom or Athlon X2. CPU clock speed may not buy you as much as fast disks and RAM.

I downloaded the netboot version of 6.2RC2 some days back and was pleasantly surprised to find that almost all the hardware was correctly recognized. This is a 2 year old compaq laptop with an Ralink PCMCIA wireless card. Not even the latest Linux distros can detect this card but OpenBSD and FreeBSD have the excellent ral driver in the kernel. Moreover the configuration is so simple when compared to the mess in Linux (iwconfig,iwpriv,ifconfig??) not to mention the troubles I had with ndiswrapper

All the BSD's use X.org anyway nowadays, so the folks who are looking for a good GUI environment won't be disappointed. Again, the laptop display settings were correctly detected and I didn't have to touch xorg.conf at all

Give OpenBSD and FreeBSD a try - you won't regret it. Having said that, prepare to actually RTFM in case you run into problems. 99% of the time the answers are in the fine integrated documentation that comes along with your OS install.