You should not play games over wireless but many of the steam games are not really online games like first person shooters that are extremely sensitive to any delays.
Pretty much anything is going to be better than the old router you have. I would look at a mid priced 802.11n router dual band something like a asus n53 or a tp-link wdr3600. There are many other these are just examples. This should leave you money to upgrade the nic cards in your PC. If you have old 802.11g nic cards it does no good to upgrade just the router.
Still it is likely your current router and nic cards can max your internet connection. You need to run speed test and see if you can get the 25m if you can then you will likely see no significant improvement by replacing the router.....it is getting very old and outdated though.