I have a USB microphone and using a laptop?

I have a pretty basic set-up. i have a samsun co1u condenser microphone it is a usb microphone and not xlr. It is surrounded by an SE electronics reflexion filter, the mic is mounted on a mic stand with a nady pop filter and shock mount. i have it ran to my laptop via usb cable. i have a toshiba satellite l305. it uses a realtek onboard soundcard that was standard. 4gb of ram, 300gb of memory. 64 bit operating system. I am using Mixcraft 5.1 to record from. On a vista laptop. My question is i am seriously considering purchasing an aftermarket sound card and a mixer. which would be more practical for my situation to improve sound quality an onboard soundcard or usb. And Why? Also I need a mixer that has a usb input. which is a good mixer for this type of set-up? Is there any other fairly inexpensive things i need to buy to make this sound alot cleaner. I am trying to go for semi professional sounding recordings for pop, hip hop, and rap music. anybody have any tips that can assist me in getting that premium sound. it sounds decent now, however i will cut the monitor on for the recording so i can hear myself thru my headphones "Behringher HPX2000" but theres a slight lag. i am trying to eliminate that. I believe a better soundcard and i belive i will need a mixer also. Any tips would be greatly appreciated.