If you've bought a HSDPA contract (R599/ Month, for 1 Gig, etc.), you should get HSDPA speeds (1800/384 max).
But if you've used only 3G before (with a 3G phone, for example) or are on pre-paid, the systems would automatically put you on the lower tariff and limit your speed to '3G' speeds (384/64).
This setting is applied to your mobile number, so if you want the HSDPA speeds, you need to call the call desk on 155 and get them to 'upgrade' you to HSDPA.
I've found the best way to check which speed-profile you're on is to run the Vodacom speed test and look at the up-link numbers as there should be less congestion in the up-link to confuse the results. If you get higher numbers than 64K in the up-link, say 100K+ you must be on HSDPA.
But if you constantly get just under 64K (55-57K), your on the 3G profile and can upgrade.
Both feverdj (370/55) and timvdwest (381/56) are getting full 3G speed, but obviously not HSDPA.
Make that call, you know it's the right thing to do!!