OMSA cannot update drivers. You need to use the Update Utility (be sure to follow the Installation Instructions found on the download page for each ISO):
www.dell.com/.../poweredge-840
You could also just reinstall Windows using SMTD, which will install all needed drivers along the way (same thing, follow the Installation Instructions):
www.dell.com/.../poweredge-840
Boot to SMTD, follow the prompts to install the OS, then insert the 2008R2 DVD when prompted.