ATI's Radeon HD 2600 cards in AGP flavor are set to appear in the first half of September. German and Austrian etailers have already listed a few GeCube cards. Cyberport.de claims it will have GeCube AGP cards on stock on September 6, as you can see here.
A GeCube HD2600PRO (GC-RX26PGA2-E3) with 512MB DDR2 memory will cost you from €104 to €122, while the 256MB version is just slightly cheaper, ranging from €99 to €109. Needless to say, it pretty much makes no sense to get the 256MB model at these prices, since you'll save as little as €5. For comparison, you can get GeCube's PCIe HD2600PRO with 512MB for €82.80. On the other hand 512 MB won't bring much if any performance increase over 256 MB cards.
Although they cost about 20-25% more than their PCIe siblings, ATI's AGP cards should do well, since they're still pretty affordable and Nvidia has no DX10 cards for AGP. If you want to turn your old AGP machine into an HTPC, HD2000 cards are the way to go, as you'll get UVD, HDMI and DX10 for peanuts.