Well if you have a Celeron that means that you have a socket 478 intel motherboard. You will only be able to put intel processers into that, so if you go AMD you will be looking at a whole new motherboard. The other thing you need to think about is that if you plan for a high range P4 (650 or any other that supports 64bit) the chipset (thing that interperates the processer) will not support that technology.
Also if I know my Compaq's you have an AGP 8X slot. Now this is a mixed blessing. Its good as you can get a range of top end cards (ATi's X800 series and X1K series + every nVidia card except the 7800GTX) however it will lack in data transfer speed, so you will not achieve the quality of someone who has PCI Express X16
Now you mentioned a RAM upgrade. Well seeing as you only have 256MB of slow PC2700 or slower RAM its going to be time to upgrade that NOW!!!! Your processer and graphics lack, yes, but your intagraded graphics chip is robbing your systems memory of its resources. Solution Corsair 1GB kit
http://www.newegg.com/Product/Product.a ... 6820145440$80 includeing three day shipping. You will notice a huge preformance boost in all areas by installing this kit.
Personally if I were you I would think twards building a new system.
AMD Athalon 64 x2 4000+ / Intel Pentium 4 650 Prescott
DFI LanParty Motherboard
1GB Corsair XMS RAM
Plextor or Lite on DVD +/- RW Drive
ATI Radeon X800XT PCI-E/ nVidia 6800GT PCI-E
Western Digital Cavilar 160GB SATA Hard Drive.
Case + power supply of your choice
Now I dont have the newegg window open but a system there abouts will run you $1500 ball park. This may seem like a hefty price tag, but lets review. Both processers are 64bit enabled. You will be able to go right out and buy Vista once it comes out, while people like me will have to get a new processer. The LanParty series of motherboards are amazing. Increadably easy to overclock, very user friendly, and have had few problems that I know of. Corsair and OCZ are probibly the highest pillers in the RAM world. I chose Corsair XMS as its fast, and relatively cheap (I have a 1GB kit with 2-3-3-6 cas timings and it cost me $130 so you can see its a bargin.) We then move to your DVD drive. You can go with pretty much anything here, but im a Plextor fan boy, so I suggested that. Graphics. ATI = Sapphire. nVidia = BFG Tech or ASUS. Both cards are about the same in all fields. The only difference is that the X800 benches about 200 more 3DMarks than the 6800 does.
Well I hope I didnt confuse you too much. Congo feel free to nit pick.
Cheers
Cameron