If it's truly a client so that the TDM and license serving is done elsewhere then:-
CPU:- I have know recent experience with Pentium processors, but as long as the quoted 3.0Ghz is a genuine 3.0Ghz, as with the Xeons, then I see know problem with the entry level processor for this machine.
RAM:- This (and Graphics card RAM) is where the user will see the most performance benefit. Depending on use you may get away with 1Gb of RAM. If however you're expecting to manipulate large assemblies and generate complicated geometry there is a performance increase to be had from upgrading to 2Gb.
HD:- Without the TDM, 80Gb SATA will suffice. I would configure the Model Files to be on the local machine and ensure the users know that their data is not protected from HD failure unless it is Checked back in to the TDM. I don't think that a user would notice the difference between a 7,200 RPM and a 10,000 RPM spindle speed so keep your money.
Graphics Card:- Remains unchanged, I just love the Quadro FX1400. But if you really need to save the pennies then I see no problem with the FX540. I'm not mentioning the ATI purely because I have no experience of it's compatability with I-DEAS.
Network Card:- If you've now got a separate TDM Server then having Gigabit connectivity will see a marked performance improvement. But for that, everything between the client and the Server needs to be Gigabit capeable.
Hope it helps.