Assuming that Oracle and OS's licenses are not a problem for you , sure you can!
Once you've chosen a supported OS (eg. RHEL4) you can follow Oracle documentation for deploying your RAC environment. If you want to play, you can also use an OS that is not supported, although you may encounter problems for example during linking of Oracle binaries (you may fall into big headaches!).
My personal installation runs very well on my laptop:
2.6.24-gentoo-r8 #1 SMP Tue May 13 18:38:54 CEST 2008 x86_64 Intel(R) Core(TM)2 Duo CPU T7700 @ 2.40GHz GenuineIntel GNU/Linux
However it's not a RAC. I've never installed a RAC on OSs different than RHEL and Solaris.
Given your system, I would recommend installing x86_64 version for both OS and RDBMS, only don't try 32 bit Oracle on a 64 bit OS.
The only option you have for shared storage is using ASM or OCFS (very buggy). Since it's a test system, you may also try with a shared mount via NFS, but if I remember correctly it's not supported (don't know if it works the same!).
Oh, you also need two physical NICs on both servers for public addresses and private interconnect. These are required for proper installation of clusterware software.
One last word: maybe it is better to try such a complex environment on virtual machines rather than on a real hardware! For testing purposes you may be able to startup two VMs with 512MB of RAM each and be able to install correctly a Linux system and a small Oracle instance.
For rac, you are going to need several separate shared disks between the two servers.
You need at least 1 OCR and 1 Voting disk.... no bigger than 100M each. Best practice is to have 2 OCR and 3 Voting, but this is a test environment so that is a bit of overkill. Then you will at least need 1 shared disk for data. My suggestion is to use ASM over ocfs. ASM is far better and most big RAC environments that I have worked on use it.
As much as I love Gentoo, RedHat seems to work best with Oracle. You might want to look at CentOs as the OS to run.
There are pretty comprehensive guides to running Oracle on RedHat here