idc said:if the sound card in your PC is good enough to do the work of the DAC you can get a minijack to phono cable to connect PC to amp. I would try that first as it is the cheap option. Or you can use the USB out to send a digital signal to DAC, bypassing the sound card and then phono cables to the amp.
I agree with idc. First get some speakers if you haven't already the RB61 II. Next is the amplifier, something along the lines of the Denon PMA 250SE will be great or something similar second hand depending on your country. I've run my Klipsch RB81's with this amp fine as the Klipsch speakers are quite sensitive and don't need a lot of power for decent volumes. You can pick the Denon up for about £40-£50 if you don't mind second hand bargains. Connect your PC straight to the amp, if this doesn't sound good enough you could try the Asus Xonar range of souncards, very nice sound. If still you want improvements go for separate DAC. If your going for a new amp, may be worth looking at amps you can connect a sub up to. For cables, try sticking to fairly thick copper types but you don't need to spend a fortune.