OK, so I've found me a nice Pentium 4 (don't really know what kind yet, haven't even plugged it in yet) ThinkCentre and I'm thinking of pimping it out with an old X1900 card but I don't really know where to start. Business desktops are usually a pain in the butt to upgrade for gaming/entertainment uses, and I'd like to know if you guys have any experience with this, and also I'd like to know whether or not a P4 processor would "hold back" the performance of a X1900 or even an 8800 GTX.

The Pentium 4 is like XP - it was around for way too long, though for different reasons. The P4 because the name now says nothing at all as here are more iterations than you can count and XP because it became obsolete about half-way through its lifetime. Is there a model number printed anywhere on the case? That way we may be able to find more info on it.
Wether it'd hold the graphics back or not s really a matter of wether it's a recent P4 or an old one. A Core 2 (Solo|Duo|Quad|Extreme) would probably be better no matter how you looked at it, but a P4 EE or dual-core should still hold up decently whereas one of the first can probably be discarded as garbage.

Here is an analogy of what pimping out a Pentium 4 - based laptop with a X1900 card would be like.
http://www.roadtransport.com/blogs/truck-and-van-blog/ugly_car%5B1%5D.jpg

A P4 with a PCIe x16 port? Are you certain of that? I don't remember ever seeing that combo and you'd need that for a modern video card.
Frankly I think it'd be a waste of money upgrading it, I'd repurpose it as a file server or dedicated download machine

Well, if not I'll just pay a little bit of money for an AGP card that ATI still seems to be making.

It really depends on the memory and processor. If you can bring it up to 2gb memory, and it has a decent processor, you should be ok.
I bought a custom built Pentium 4 PC in 2003 and it lasted me til literally this time last year. I upgraded the memory, hard drives and graphics card and was able to happily play through Bioshock on medium settings. Crysis was playable on low. For some reason, Vista was too much for it and I had to stick with XP most of the time.
I eventually self built my current one because in the end, it just died.

My secondary gaming machine is a IBM (Lenovo) ThinkCentre. It has a 3 gig HT P4 with DDR2 ram, 80 gig SATA HDD, and a PCIe x16 slot. It only has a 300w power supply. I upped the ram to 2 gig, put in an extra 500 gig HDD and put in a HD3450 256 meg video card. This card was designed for low power and doesn't use an extra power cable. It runs GOG games great. As a matter of fact it will run anything from 2005 and older pretty well. And it will run newer games that don't need much hardware. I even managed to run Fallout 3 on it (800x600 with everything set to low though, LOL). It didn't cost too much to upgrade ($15 for the ram, $55 for the HDD and $30 for the video card).
If your going to use a X1900, you'd probably need to replace the psu. I don't think a 300w or lower psu will work with that.
PS: This machine is built like a tank. It should last for quite some time. IBM/Lenovo machines are pricer, but well built.

Good to know then. Is it a pain to fiddle with the hardware inside? Is it like Dell where they're all proprietary and all that?
I've even seen some Youtube videos with very, very smooth gameplay on medium to high on games like L4D2 Demo, TF2, Rainbow Six, Borderlands, etc. Which is great, because I just want a computer for my big TV in the living room. Gaming is secondary to me, but it would be pretty cool. I see that the 8400 GS is a good cheap card for that, and if I intend to pay more, Sapphire's got some AGP cards that are damned good.

They did exist, the Fujitsu-Siemens Scenic P320 and Esprimo P5915 are two examples.

Cool, I'm thinking of going AGP (after all, almost every computer made before 2006/007 HAS to have AGP) since it uses less power, if not I'll get a PCIe card and a larger power supply, and that's if the PCIe card will even require one! This is damned helpful and I'll just mark your post as the answer, though if people have more info that would be nice.

So not this machine have an AGP slot in then? I have another pc hooked up to a 37" LCD tv. It's great watching videos on it. If you go with a wireless kb/m, find one that has good battery life for the mouse. Mine always seems to die fast.

Dell GX280's have a PCIe x 16 slot too.
michaelleung, my Thinkcentre was built around 2005 and it doesn't have an AGP slot.

Well, I'm thinking of wiring up a Xbox 360 controller and some sort of 10 foot interface for games on it. Maybe Boxee.

A couple good cards are either the HD 4870 or the 9600gt green edition for PCi-E if you want cards that don't draw much power underload. The HD4870 draws 75 watts of power I think and the 9600gt only draws upto 59 watts of power. They both don't require the extra 6pin pci-e power connections either. Good cards if your thinkcenter is actually pci-e and you don't want to upgrade the PSU to handle something beefier.

Just curious, would this upgrading work on a slimline PC?