For Half-Life2 more then likely the fastes processor you can buy will be your best bet with a fairly decent vid card.
For Doom3 the fastest vid card you can buy will likely be your best bet with a fairly decent processor.
Half-Life2 I would expect should run fairly decent on mid tier(by AT standards) rigs even at decent settings, should play fine on the high end machines. With the physics engine they are using I expect it to be a bit more processor bound then vid card until you really start to crank things up. Judging by the visuals it appears that at most it is using some DX8 level features and Valve has already said that the minimum vid card requirements are TNT2 level boards(although obviously with low detail).
For Doom3 the game mainly revolves around the DX7 level feature set, but makes decent use of some DX8 level shaders but at the highest end it exceeds the DX9 feature set(the game supports FP32) however based on Carmacks comments at that point you won't be able to pick out the differences easily(if at all, comparing FP16-FP24-FP32). So, in a theoretical sense at least, you would need a FX board to run Doom3 at the 'highest' detail settings, although even those with FX 5900 Ultras are likely to run the game in FP16 so any DX9 level board should be able to handle D3 on a feature set basis at least. The game is shipping with a NV10 code path, so even the original GeForce should be able to run the game(lowest detail settings, likely a fairly low framerate too), out of the current boards and based on what we have seen it appears that the FX5800/5900 series boards are the best for Doom3 at the moment(makes sense as the game will utilize heavy stencil fill loads which is where the FX58/59 will be at their best relative to their peers).
On the RAM end, I would say that 512MB is likely going to be the sweet spot for both Doom3 and HL2. HL2's large environments will likely benefit from exceeding 256MB of RAM although it wouldn't surprise me if Valve did have a fairly well optimized level break up/loading scheme as they pulled it off quite nicely in the original Half-Life, I'd still adivse 512MB. Doom3 is going to be cramped in comparison to HL2 but the texture load appears to be significantly higher then anything we have seen to date comparing like sized environments. A texture load of around 80MB is being talked about, enough to make it advisable to move to 512MB if you haven't already.