Help installing 64 bit

Ok here it goes.....I did a complete build a few months back. I installed Vista 32 bit OEM and everything is fine. When I found out 32 bit still had the RAM cap, I was bummed cuz I have 4gig installed. So I decide to go 64 bit. Now here is my problem. I have tried everything I can think of to install 64 bit, and to no avail, so I ask for assistance in this matter. I purchased a 64 bit OEM cd. I can not for the life of me get this to install. I get the boot from disk at startup, and it runs through loading files, then after that initial go, it just loads the 32 OS off my HDD. I have gone as far as formatting my second HDD, used for Raid 1, unplugged my HDD that has 32 bit loaded on it, but still no avail. I am at a loss, any help, suggestions, reasons to why, would be much appreciative. I thank all of you in advance who assist me on this.

Well, I found the issue. After wasting 9 hours doing a DBAN on my 2 HDD's, that wasn't the issue. The issue is RAM discussed here http://support.microsoft.com/kb/929777 It is an error with x64 and RAM that will cause it to fail the load/boot, if you have more than they say 3gig, I say 2gig, of RAM installed. So, you have to install windows with only 2gig of RAM then go to the link listed above, and d/l the hotfix. Once you install that hotfix, you can reinstall the extra RAM you have. Thx again Emp for you thoughts.