So, I have been using a D-Link G510 since I got my computer (3 years about.). Now playing games online is just a lag fest. What can I buy / make / use to extend my wireless connection?

Is it the speed of your wireless is slow, or you need longer range on your wireless? Wireless connections are not recommended for online gaming anyway, as they are slower than using an ethernet cable, not to mention the dropouts you sometimes get.

Some brands have got some more expensive "ranged" cards and routers/AP's that use the same bandwidth but can go further. Perhaps you should look into some of those.

I always use wired connections for gaming.

They are faster because they dont have to be encryted / decrypted.
Another advantage of the wired connection is that the speed is more constant (10M or 100M or 1000M).
In most of the cases even 10M wired is faster then a 54M wireless.
When you are using ADSL the speed of internet is most of the cases 4M Download / 512 k Upload.
So 10M is more then enough unless your are having a (small) LAN party

So if you can go wired.

Otherwise try turn of the encryption and your connection will be faster but less secure.
These day the wireless connections go to 108 Mbit/s and even faster, but for gaming is the speed no that important only the delay (visible with the ping). For tests on delay you can always use a ping from a DOS window.