I agree that the Asus PCI TA is excellent, (& cheap from Scan at about £16). Outperforms my Eicon DIVA external isdn modem too (via ethernet).
Don't think Clara can give you an unmetered 128k account - I eventually went for the 64k dial-up, as the business package offers 360 hours per month, with no cut-off (till you reach the limit).
There are 128k ISP's out there, but I've yet to see any without a fair usage clause. Allowances tend to be 200 hrs at 64k or 100 hrs at 128k. You could try looking at ISP's such as Vispa for the 128k service (tho' I found that my telephone exchange suffered from line drops etc).