The Inspiron 6400 has wireless n and both 2.4 and 5 GHz. This will enable the option of 5GHz for least interference and possible higher speed while retaining 2.4GHz if distance is a problem (5 GHz doesn't travel far).
The Dell 15r doesn't have 5GHz but it does have wireless N so you should be able to get good speeds.
The two killers are interference from other networks and distance - especially if through solid walls.
Having devices on the network using old technology (especially wireless B) does slow down the entire network.
BT Infinity 2 - IP profile 77 / 20 - super fast!
Previously BE Unlimited - 21,000 Download 1,200 Upload but then moved house - 6,500 Down, 1Mb/s up - gutted!
Ex <n>ildram , been to SKY MAX - 15,225 Download