The maximum ram you can install is 8GB of RAM and that should only cost about $50.00 to upgrade. You can always sell the older 4GB kit to make the cost of a ram upgrade for a 2009 Apple iMac even cheaper.
When it comes to hard drives you have two options – you can replace the 160GB drive with a larger hard drive like a 1TB hard drive and this should only cost you about $50 to do. But if you want a faster mac and only need a bit more room than the stock hard drive then you should upgrade to a SSD drive. The 250GB SSD from Samsung is a best selling SSD and sells for under $100.
If you do install a SSD you will need a 3.5 inch to 2.5 inch adapter. I recommend the icy dock model as there are no tools needed.
It does take a bit of work to get to to the hard drive to replace it on this iMac model but it’s not impossible.
Here are a few links to help you with RAM and hard drive replacement on the 2009 apple iMac:
Install Memory in an iMac – Apple
2009 iMac ssd replacement – 9 to 5 Mac