One of the systems that supports CIFS and NFS perfectly (and I currently own one) is Synology.
Just works, no complaints.
Any other NAS that supports either CIFS or NFS will work because you can mount any of these protocols in Ubuntu without any problems.
One word of advice I have is to just check the website forums page of the brands you are interested and have a look for any NFS or CIFS complaints users are posting there.
ie: Synology had a problem with CIFS low transfer rates that was fixed 2 versions of the software ago, you can read about all the complaints on their forum. Its fixed now.
To use a NAS on your network cannot be more simple:
1) Plugin the NAS to your router, it will probably be setup to look for IP addresses by it self
2) Create users and shared folders on the NAS using the NAS interface (much likely a HTTP page)
3) In your Ubuntu system install the required protocol files (CIFS or NFS)
4) Modify your fstab file (`/etc/fstab') to mount the shared folders you just created automatic on each startup, instructions for that normally can be found on your NAS website support pages.
Thats all! Its that simple. Gl and if you need any further assistance on this just ask ;)