Thoughts
My evaluation of such low-end hardware using the latest graphically challenging games might seem pointless at first glance. Look twice and you'll see that while both HyperMemory and TurboCache boards were generally really unhappy with the latest games, analysis of UT2004 shows that in older games (and to some extent new ones at very low resolution) they'll do fine. If your aim is to build a really cheap PCI Express-based gaming box for the kids, or to use a card as a stop-gap solution before you pick up something more powerful, they do fine.Integrated graphics are outrun by both, so their usefullness increases in that respect. They're no gaming powerhouses, but neither are they totally useless. Using available system memory to augment card memory, in order to lower costs in the low-end, is a valiant endeavour and one I'm happy to see. The resulting hardware is faster than integrated graphics and has the basic attributes to do well with older titles, and new ones at low resolution.
For the £35 either will cost you, you can't really complain. It's easier to choose HyperMemory X300 SE hardware since all have a 64-bit local memory subsystem. TurboCache boards are available in 16MiB/32-bit configurations, and at prices that don't make them easy to tell apart from their better siblings.
If I were buying, I'd choose a TurboCache since performance is generally higher in the games I'd play. Beware of what TurboCache part you finally choose, though! Both do well for their price segment. Both are recommended if you need something cheap, as a stop-gap to something else or as a means to avoid integrated graphics. The Half-Life 2 performance of the HyperMemory board is worth a final mention, too.