Some factors you need to take into account
- the existing RAM may be the wrong type for the new motherboard
- you will probably need a larger capacity power supply, if you haven't already fitted one
- if you get a complete new tower, you can always swap across the CD-writer and new second hard drive - and you then still have a complete saleable system, to offset the extra cost of buying a complete new tower. There may also be a warranty benefit too.