I'm a bit confused by the details you've posted. Firstly, by "N WIRELESS ADAPTER" and "OLD G WIRELESS ADAPTER", I assume you actually mean *ROUTERS*, the wireless device that connects by ethernet cable to Virgin's Cable Modem?
A Wireless [Network] Adapter is the component installed in the PC, either an internal PCI card or a USB "dongle", which connects to a wireless network.
If a new 802.11n (N) Router is giving an internet download speed of 49Mbps, then you are effectively getting the full benefit of the 50Mbps service.
I don't understand why the older 802.11g (G) Router, if that's what it is, only gives a speed of 20Mbps, but it isn't clear what you're measuring. The Router will connect by ethernet cable to the Cable Modem at 100Mbps, a "wired" connection from one of its LAN ports to a PC will run at 100Mbps, a "wireless" connection from a PC will have a nominal connection speed of 54Mbps. Only the "wireless" connection should fail to achieve the full potential of the 50Mbps internet connection.
With an 802.11n (N) Router and compatible Wireless Network Adapters, the data transfer speed of a "wireless" connection between PC and Router should be 150Mbps or 300Mbps, depending on the hardware. The speed between Router and Cable Modem will still be 100Mbps.
If you are connecting a Solwise "wireless" adapter and only getting 15Mbps, then something may not be correctly configured, but what are you measuring - Solwise to Router, PC to Solwise or internet download speed?