DGL-4300 / Getting Wireless Boost In Specific Section Of House?

My parents house is basically an old ranch style home that was added on to with a 2 story + basement. So I can only guess at the materials between where the signal is weak and where I have installed a DGL-4300 (Dlink Wired and Wireless gigabit router). It resides in the basement which is directly under the 2 story part of the house and everything on any floor of that section seems to get a good signal.

Admittedly I haven't done a lot of extensive testing since I really don't have much stuff that has wireless connectivity, and the one laptop they have is fairly stationary sitting right on top of the wireless router 2 floors up.....signal strength is always good to it.

However the older section of the house gets a pretty weak wireless signal and now with everything being wifi ready I am going to see about solving it. For instance, I've had a hell of a time connecting an older Zune 2.0 8gb version to the wireless network anywhere in the house, I just assumed it was broken or puking on the wireless security which is some version of WPA I can't recall atm if it's one in particular or set to both. But I temporarily disabled all security to see if it would connect and the router saw the device connect with a 20% signal strength for a bit, then the Zune started puking on connecting ..again.

I figured small device, it might even be broken. But I know the signal strength to the older section is not going to be very stellar anyway because the signal on a direct path has to pass through concrete walls and then a concrete foundation......plus it has cordless phone systems to compete against in that same space is has to travel. And anything the kitchen appliances and such might be interfering with it since it's potentially in the way as well.

So, any low cost solution (less than $50 dollars) I can hook into the existing wired network and will play nice with the existing Dlink router as it's going to remain the boss since it was fairly costly and works fine. To work as an extension of the same wireless network.....

I really don't have a problem with updating firmware, connecting into the device to configure it...or even putting new software on it if the directions are decent. The only thing I really care about is that it is cheap, has the capability to fit my bill. I'd guess it'd need to cover a 50 foot circular span with a strong signal and maybe 75 feet out with a medium strength to make sure there's no too weak spots or close up any potential dead spots due to materials used in construction. Oh and it has to be very stable or be pretty stable and have a remote reboot that'll clear up issues, I used to have a linksys router I had to reboot probably once a week to keep it working but it had to be power cycled. It was in the basement, near the patch panel the house has and it got old enough that I got the Dlink to replace it even though it was still functioned besides the freezing/sluggish performance issues requiring power cycles.