If you are trying to start the new SimplyMEPIS 6.5 and it won't boot to a gui screen that may mean that it is trying to load the wrong video driver at boot.
Some things to remember to do at first boot, before you log in a demo....
1) Press f3 and choose the resolution that you want.
2) If you can't boot to a gui and you just get text log in then re-boot by logging in as root using root as a password and then re-boot. When the grub screen pops up just boot with the 'vesa' or 'small' option. You can always upgrade the vid driver once everything is shown to be working.
If booting with vesa or small gets you going then you may be able to upgrade the vid driver by going to kmenu, system, MEPIS, X-Windows assistant. If when you are tring to upgrade the video module you fail and you can only boot to the text screen then use the live cd and X-windows assistant to re-install X. Reboot without the cd and you should get a gui again. The newer nvidia cards will take the 'new' driver and the older nvidia cards may work 3-d well with the 'legacy' driver.
Here's a link to the MEPIS wiki with some good tips on video.