I think you mean you want to add the 500 GB new drive to your system. Then you want to install Win 7 somewhere so that you can have a dual-boot system with Win 7 and Vista.
First, do NOT worry about Master and Slave. You have only SATA drives, and those have NO Master or Slave - that system applies only to IDE drives. So, do NOT change or install any jumpers on pins on the back of the drives. If there are any pins and jumpers there, leave them unchanged.
You already have one drive (180 GB) installed and operating with Vista on it. To install the second drive, just mount it physically in a slot and connect a power supply and a data cable to it. Plug its data cable into any empty SATA port, probably one right beside the one already in use. When you turn on power, go immediately into BIOS Setup and look for where the SATA ports are configured. Check that the port you have connected the new drive to is Enabled. It should detect that new drive here. Nearby, look for a line for SATA Port Mode. On some mobos, you set this parameter once for all SATA ports together - if that is how yours works, leave it untouched. Save and Exit to reboot.
On the other hand, if you can set the SATA Port Mode separately for each drive, set the port for the new drive to AHCI or Native SATA mode. Do NOT set it to RAID. Save and Exit to reboot.
You will NOT see the new drive in Windows. It has not been Partitioned and Formatted yet, and those steps are necessary. However, you may not need to do these yourself right now.
I suspect you would like to install Win 7 on the new drive, not the old one. If you do, the Install process can do the Partition and Format operations for you at the beginning.
Assuming you are installing to the new drive, put the Win 7 Install CD in your optical drive and reboot so that the machine boots from the optical drive unit and starts the Install Process. Make VERY sure that you tell it you want to install to the new 500 GB Seagate unit, and I assume you will want it to use ALL of that drive in one Partition. Use the NTFS File System. When it offers to make a Dual Boot system with the existing Vista as the other option, tell it yes. Once all the options are set, let it proceed. The Full Format (the usual default option) process takes MANY hours to run before the actual installation starts, so just be patient.
When the whole process is finished, you will have a dual-boot system that offers you a choice of which OS to use at each boot time.