Before you replace any of the equipment you have first make sure there is nothing wrong with the signal coming into your house. Open up the modem configuration utility by entering the following address in your web browser: http://192.168.0.1. Your recieve power level should be between -15dBmV and +15dBmV. Also, if your power level is very close to 0, like 1, you may have a problem. Your transmit power level should be less than 58dBmV. If your levels are off, and it is not because you recently split the line, then you will probably need to contact RR and have them send a tech. out. If you did just add a splitter to your line, remove it and see if that helps.
If your signal is OK, the easiest way to check to see if your wireless adapter is functioning properly is to connect your computer directly to the modem with either an ethernet cable or a USB cable. (you will have to install the appropriate drivers to connect with a USB cable). Ethernet cables will perform better than USB cables. If your speeds go way up, you know that the problem is with your USB adapter. Also, the wireless adapter you are using operates at 11Mbs, but your modem/router is capable of connecting at 54Mbs. So if the adapter is the problem, and you choose to replace it, try to find an adapter that operates on the 802.11g Wi-Fi standard (54Mbs).
I'm sure you have probably already tried rebooting your modem, but just in case you didn't, you can give that a try as well.
Hope some of that helps.