A T61 with Intel graphics and a 4:3 XGA screen (which is your preferred resolution, I believe) should be an easy find.
These machines will take up to 8GB of RAM and run W7 flawlessly. Graphics performance is about on par with your current X300 in the T43.
Overall, they're pretty well-built and should suffice for several years...
After some thought and consideration, and an opportunity appearing, I ended up gettng just that - A T61, Intel graphics, 4:3 (1024x768), including a 160 GB hard drive and 3 GB of RAM.
Physically the machine is uncannily like the T43, but yet not completely. It doesn't feel as solid as the T43, but still solid enough. The fan is increadibly more quiet. Graphics-wise though I must say that the T43's X300 graphics wins. Running Minecraft under Linux yields fully accpetable performance on the T43 (with the ATI fglrx video driver), whereas on the T61 it is struggling, due to the superior 3D acceleration in the fglrx driver. Unfortunately, ATI have dropped support for the X300 in later versions of the driver, so I'm stuck at a slightly old Debian Linux version in order to retain the fast graphics on the T43. On the T61 I can run the latest version more or less out of the box.
Overall, I'm pleased with the "new" T61, but the T43 still has a better overall feel to it. Slimmer and more solid. Also, it's increadibly worn, most of the keys are shiny, not matte, as is the palmrest, after 7 years of daily use.