Is there any way to boost Wifi reception on my Acer Aspire One netbook?

If I want to download anything, it has to be done from my car through wifi since I only have dial-up at home, and I have found a few public wifi spots that are great, but the signal strength is low, and I have to set the laptop on the dash to get good strength. If I get a usb wifi adapter, so I can stick it to my sunroof and keep the laptop on the penger seat, will that get much better reception? I was thinking about even getting a tiny 4" dish style adapter so I can aim at the sources and get super reception strength, about $60 on eBay. Would those offer much improvement over the Acer Aspire One's built-in wifi? By the way, it uses Windows XP. I cannot get anything faster than dial-up where I live in the countryside, and live in a dead zone for the ATT and Verizon 3G internet adapters... so they only run 3x faster than my dirt cheap dial-up, so it's not worth the cost, and the satellite stuff has far too many bad reviews. I'm just looking to improve my wifi reception for my laptop. It does not have a PC Card slot on the side, so USB is the only adapters it can use.