I have successfully installed Win 7 Hm Premium on a 3 yr old Gateway 564GE, with on board graphics only.  Of course, this is a relatively new Intel Chipset.  I installed the same O/S on a 5 yr old Dell 2400, with absolutely no difficulties of any kind - until i began running certain programs.  The only screen resolution I can employ is 640 x 480, 16 bit color.  The resolution of course is not great, but generally adequate for routine, noncritical graphical purposes.  The problem is that, in the case of certain applications, the screen display characters and graphics are so large that required access to necessary buttons and controls is impossible.  I have 3 available PCI slots, and I can easily find a video card sufficient to give me the required resolution (I believe I could get by with 1024 x768).  However, all the cards I have located require a power supply rated at least 250 to 350 watts, and of course, the Dell has a romping, stomping 200 watts.  I'm reluctant to replace the PSU in such an aging machine, out of concern for unforeseen problems/conflicts with the motherboard after such a replacement.  Is there any external fix for this - such as an externally powered graphics card?  It's not absolutely necessary that I upgrade this old machine to Win 7, but it runs so well on the machine (except for the graphics), that I'd like to do it if economically feasible. 

Any suggestions will be appreciated.I'm afraid on a Dell 2400, what you see is what you get...
Post the full specs...but i doubt it'll get much better...especially if it's a Celeron...Thanks for contributing to the cause, PATIO.  I wouldn't spend a second trying to upgrade any Celeron powered PC.  This is a 2.66 mhz P4, with 1.5 gb RAM, and new, midsize (320 gb) Hd Drv.  For an old unit, it's okay.  I really don't mind getting an inexpensive ($40 to $60) video card.  But as stated above, I'm hesitant to undertake replacement of the PSU.  Since the PSU is so underpowered, I haven't come up with an alternate route.Unfortunately adding a vid card will require a new PSU...perhaps you'll find some happy medium there...Will a standard ATX power supply fit in that Dell?  If so, here's a low cost PSU that might interest you:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817171046

And, a video card suggestion:
http://www.newegg.com/Product/ProductReview.aspx?Item=N82E16814187042Soybean:

Thanks so much for both suggestions.  I'll check them out.Especially for Soybean & Patio (but also for any others who may be interested):

After thinking about this, and exploring all options, I remembered that I had an old, unused Compaq XP machine which USES an NVIDIA GX 5200 PCI card.  This is not the more recent GX FX 5200 card.  It is probably 5 or 6 years old.  I took the card out of the Compaq, installed it in the Dell 2400, and it gives me all the resolution I need - up to 1440 x 900.  So far, it appears that the power supply is not being overtaxed, but time will tell.  I have tried (unsuccessfully) to find documentation on this card in order to verify PSU requirements - so I'm just keeping my fingers crossed.

Again, thanks for all the helpful suggestions.A good low-cost solution...well done !