I have a Linux PC with a single IDE hard disk (160 gb).
I want to use half of it for file systems and half as raw partitions. Is it possible with single disk? If yes, then how to I do it when I already have 70 GB allocated as filesystems?

Thanks.

raw means unformatted unpartitioned. I don't think that you meant that.
If you already have a 70 GB partition on the drive and you want to partition the remaining raw space then use "fdisk" or "cfdisk' to partition it. Then format it with a file system like ext2, ext3 reiser ect. Then mount the drive. Then you can use the space for whatever you want. You can mount it manually or in /etc/fstab if you want it auto mounted. Read the man page for fdisk and mount.
$man mount

If you mean that you want to leave it raw then don't do anything to it.

what use would raw disk be?

Thanks guys. By raw, I meant if I could bind the unused/unformatted space to /dev/raw etc. I am planning on Installing Oracle on this box and use raw devices to store datafiles.

You need to use DiskDruid or another disk partitioning tool to create the partition that you want without formatting it ( no ext{n} or fat ). You will want to create a sym link to the raw partition so that you can refer to it. See High Availability with RAC,Flashback and Data Guard by Hart and Jesse (Oracle Press) page 109 steps 7, 8 and 9 for configuring raw partitions. Basically, you create an extended partition for the entire disk space that you want to use using fdisk <device name, e.g. /dev/sda> . Then you break it up into logical partitions.

To use the space it will have to have a partition on it or put the space into an already existing partition. It will have to be formatted too. The /dev folder is either in / or /usr ect. depends on how you partitioned your drive for Linux. If you want to make a partition in the free space and mount it to /dev/raw I think that you can do that. Actually I don't know if you can mount that to /dev/or not I've never tried. If you can
Make the directory first
mkdir /dev/raw
Then mount it for example
mount -t ext3 /dev/hda4 /dev/raw
Look at the man page for mount. Put an entry in /etc/fstab to mount it automatically. Depending on distro it may find the new partition and auto mount it upon booting.

Anyway to use the space you'll have to have a file system on it.

what distro are you using? I ran into the problem with FC3 that rawdevices is not supported anymore, FC3 uses O_DRIVER (?) . I also am trying to set up ORacle with ASM (requires raw devices). let me know if you are using FC3 and are able to enable rawdevices. I couldn't. found a link thru google searching for linux rawdevice but couldn't get it a revised kernel to load the raw driver. am going back to FC2 (2.6.6-1.378) which has firewire and raw.