Valve will soon integrate a Steam Wallet into digital distribution service Steam.
You can fill it with lumps of real-money funds: £4, £8, £18, £35 and £70. Valve's sticking with real currency, not creating a Steam Points system.
Crucially, these funds can be bought with prepaid cards and promotional codes as well as with credit cards.
But whether you'll be forced to stick rigidly to those funding tiers even when buying a £25 game, for example, we do not know.