Folks,
Based on the scores I've generated on a T42 and T42p, and the scores I've seen posted for the X300, I wouldn't trade up or exchange a T42/T42p for a comparable T43/T43p with X300. While the X300 hardware specs are, on paper, superior to the MR9600 (same core for the FireGL T2), there is always a huge difference in theoretical v. real-world numbers. In fact, IBM and other OEMs tend to down-clock both core and memory in many implementations. The specs ATI gives are what OEMs would implement, assuming no power or heat constraints. And as we all know, that is never the case.

And even ATI's marketing literature indicates that the X300 is the entry level thin and light solution. From what I've seen, the X300 is slightly below to slightly above comparable MR9600-based solutions in terms of benchmark results based on various clockings/implementations. Only an X600/X700/X800 solution would compel me to upgrade from my current FireGL T2, and even then I think I'd only be swayed by the X700 (I've heard and seen very good things about this GPU).
Not sure why your scores for the MR9600 w/64MB are so low. Here are my results from my tests (also have my T42p results in my sig) and the MR9600 compares favorable with the FireGL T2, which makes sense, given the same GPU cores, only different drivers and amount of VRAM.
2373-CYU (1.8GHz, 1GB RAM-Upgraded, 80GB 5400RPM, MR9600/64MB):
3DMark01 (3.3.0) - 9609
3DMark03 (3.5.0) - 2617*
3DMark05 - N/A (didn't run this when I had the T42 at the time-last summer)
2373-GRU (1.8GHz, 1GB RAM, 60GB 7200RPM, FireGL T2/128MB):
3DMark01 (3.3.0) - 9629
3DMark03 (3.5.0) - 2629
3DMark05 (1.1.0) - 971
I conducted these tests on the IBM Preload, and had system restore, indexing services, and networking interfaces all turned off/disabled. Power management was set to High Performance.
From all indications, my FireGL 'smokes' the X300 64MB variety. Wouldn't even bother with the X300 since it only comes in discrete, 32MB and 64MB variations. Anyone with an MR9600/FireGL T2 would probably be best served waiting, especially since the amount of VRAM is still more important (for games and high texture CAD stuff) than a modest boost to core/mem speeds and a couple extra pipelines. And as for the PCIe v. AGP throughput, most games and apps don't tax even 4X AGP at this point, so the bus throughput for the time being isn't a necessity. Tom's Hardware did a good article on why PCIe is being pushed so hard... it's really a solution looking for a problem... hence, the big marketing buzz about digital/HD video, etc, needing the PCIe throughput (sorry to digress).
Anyhow, with the prices that T42's are going for with Employee/Shareholder discounts, I wouldn't even think twice about purchasing a T42. If you really want the next-generation notebook with enough future-proofing technology to justify the IBM premium, wait for the dual-core Pentium-M ThinkPads with SATA drives, and minimum 128MB VRAM GPU implementations, late this year and/or same time next year.
And one final final final point... lol.... The T43 doesn't offer all that much in terms of 'other' technologies... a few hundred more Mgz for the CPU, faster FSB, DDR2, PCIe (only the GPU and the ExpressCard slots utilize the PCIe throughput), no SATA (chipset supports it, but there's no way to connect a SATA drive unless it's an external USB/Firewire variety lol), no Azalia advanced audio (supported but not implemented), and what do you have to do to get all this? Pay about a 20-25% premium over similarly spec'd T42's and give up anywhere from 20-40 minutes of battery life. Oh, yes, you will give up battery life for those precious few extra seconds the CPU/FSB/DDR2/PCIe shave off Windows/Word/Doom3 loading. lol

And as for Longhorn, the T42 is plenty powerful enough to run the GUI. DirectX 9 and 64MB VRAM are minimum requirements for the 3D environment, but the standard GUI will be available for legacy/slow machines.
Having said all that... I love my ThinkPad and I support IBM and understand the trade-offs they make in terms of battery life, heat dissapation, performance, stability, made for 'serious' work not games, yadda yadda. But when it comes right down to it, IBM has always made incremental updates to the T Series. And this one is no different. There are technologies included in the T43 that are worth getting excited about, just not for the price you'll pay... The T41's were a great deal when the T42's first came out, and the T42's are now a great deal as the T43's ramp up. Wait until October if you want a great price on a T43... of course, the next must-have technologies will be in the T44/T50/T100 or whatever Lenovo/IBM chooses to call the next ThinkPad.
Daniel
* The scores for 3DMark03 (3.5.0 and 3.6.0) and 3DMark05 (1.1.0 and 1.2.0) are the same (see the Futuremark site for details). So I'm not sure why the score for the 2668 and 2378 are so much lower than my scores.
MacBook Pro 15" Retina Display / 2.6GHz Ci7 / 16GB DDR3/ 512GB SSD / Mac OS X 10.9.3