Okay, here's what I'd do:
1) Keep your speakers. You can replace them later if necessary.
2) Keep your monitor. A 19" CRT sounds nice. You probably won't need to replace that for years.
3) Keep your Windows XP copy. You can download or order SP2 for free if you need it.
4) Keep your 40GB hard disk. It will get you by, and you can always upgrade down the line.
FROM NEWEGG, GET:
5) *Either* the NEC 7170A-0B OEM, Samsung SH-S182M/BEBN OEM, Philips PHM-165P6S retail or the Lite-on SHM-165H6S retail. The drives themselves are pretty much the same: They all burn the seven recordable formats, and they all do it so fast you probably wouldn't notice a speed difference between any of them. Here's the differences: NEC, no lightscribe, no software; Samsung, with lightscribe, no software; Philips, no lightscribe, with Nero; Lite-on, with lightscribe, with Nero and WinDVD. The four drives are only about $7 apart, so make sure you get the drive with all the features you need.
6) Rosewill R5717-P BK. This is a nice-looking computer case with front USB & audio ports as well as an included 120mm cooling fan. It will work very well for you when you upgrade.
7) Sony MPF920 Black. A floppy drive is very useful for BIOS updates and ferrying small files between PCs. It should last you many, many years.
8) Zonet ZFM5620-MT. I highly recommend you buy a 56K modem like this one and keep an AOL disc handy (you can find the latter free online or from your local Best Buy). That way, if your high-speed net ever goes down, you can log in for a "free trial" and still have at least some internet access.
9) FSP Group ATX400-PA. This is overkill for what you'd get now, but you can keep it when you upgrade to dual core. It's a nice PSU for a cheap price.
10) SpecResearch KB-558BB/P. This is just a cheapie keyboard & optical mouse combo. You may or may not want to upgrade later on.
11) Corsair VS512MB667D2. This is only 512MB DDR2-667, but you'll be able to keep it when you upgrade (presumably to 2GB) later on.
12) ASUS P5LD2 R2.0. This, like the PSU I recommended, is overkill for what you're getting now, but it has all the features you'll need for a Core 2 Duo system later on. It's a great overclocker, has four DDR2 slots and 6 PCI/PCIe slots. In other words, it should last you for a couple of years.
13) Intel Celeron D 331 retail. Though it only runs at 2.66 GHz, you should be able to bump it up to close to 4 GHz with the ASUS board. It's cheap, and it should get you by till you're ready to upgrade to a Core 2 Duo.
TOTAL (with NEC): $354.23
with Samsung: $356.72
with Philips: $357.22
with Lite-on: $360.72