P.S : The below discussion is considering AMD/ATI into frame.

AMD Phenom has many variates the latest belonging to family(16) model(4) stepping(2) with flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm 3dnowext 3dnow constant_tsc pni monitor cx16 lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skin.

Most of the applications today are written with clock frequency of the CPU in the picture rather than core, there are lesser number of application designed for 64bit when compared to 32bit processor, luckily GNU/Linux has 64bit support application and the OS is designed for 64bit also.

Issues that may arise using a 64bit processor with more than 4GB RAM:

1.Proprietary Graphics Drivers for Radon GPU.
2.free -m is not showing more than 2GB.

The probable solutions that must fix these issues are :

Solving the Graphics Drivers issue involves there steps :

1.Download the required drivers from : Drivers

2.From the virtual terminal :

$ ./ati-driver-installer-9-8-x86.x86_64.run --listpkg

$ ./ati-driver-installer-9-8-x86.x86_64.run --buildpkg distro/version

 example : $ ./ati-driver-installer-9-8-x86.x86_64.run --buildpkg Ubuntu/jaunty

3.Reboot

4.From normal GUI login, in terminal ./ati-driver-installer-9-8-x86.x86_64.run

5.Reboot and you are done.

P.S : In case of trouble do an aticonfig --initial and reboot.

Fixing the RAM issue is pretty simple:

Just do this :
 
1.Check the current RAM using $ free -m then do the below, restart and cross check now free -m must show 4gb or more of RAM you have.
$ sudo apt-get update
$ sudo sudo apt-get install linux-headers-server linux-image-server linux-server

Here is video of my machine AMD Black Edition - AMD Phenom II X2 3.1 GHz Processor, 4GB RAM, 12USB, supports 6 ESATA, 2 PCI, with HDMI and 1GB Radeon HD Graphics card. Eclipse IDE starts in 3sec :O) !!