usb won't be able to provide you with enough speed to capture some of DVB-T transmission so internal is a must, there's no need for mpeg-2 hardware "decoder" beacuse there isn't anything like capture to my knowneldge, nowadays PC's don't have any problems to play mpeg even with highest bitrates and almost all new graphic card support mpeg playback in hardware mode (some of them h.264 aswell).
I heaven't heard too much about dvb-t cards/software so you might want to do some research to find out card that can work with some other software then producer provides (which in most cases limits it's capabilities).
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