How can I best upgrade my Power Mac G5 Quad?

I've got a Power Mac G5 Quad and I haven't made any hardware upgrades since I bought it 4 years ago. I want to make this computer last as long as possible so I was hoping someone on this forum could help me figure out the best way to upgrade my PC. Also, please provide me with links to where I can get the hardware that you recommend. The biggest issue I'm having is with my internal hard drive - It's only 250GB. So I've had to depend on my 1TB external hard drive a little more than I'd like. I've gone ahead and added all my computer specs for you to look at. Thank you so much in advance!

Model Name: Power Mac G5 Quad
Model Identifier: PowerMac11,2
Processor Name: PowerPC G5 (1.1)
Processor Speed: 2.5 GHz
Number Of CPUs: 4
L2 Cache (per CPU): 1 MB
Memory: 4 GB
Bus Speed: 1.25 GHz
Boot ROM Version: 5.2.7f1

Graphics - NVIDIA Quadro FX 4500
Chipset Model: Quadro FX 4500
Type: Display
Bus: PCIe
Slot: SLOT-1
PCIe Lane Width: x16
VRAM (Total): 512 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x009d
Revision ID: 0x00a1
ROM Revision: 2149
Displays:
Cinema HD Display:
Resolution: 2560 x 1600
Depth: 32-Bit Color
Core Image: Hardware Accelerated
Main Display: Yes
Mirror: Off
Online: Yes
Quartz Extreme: Supported
Rotation: Supported
Display Connector:
Status: No Display Connected

Memory - 4 GB in use - 4 Empty Slots
Size: 1 GB
Type: DDR2 SDRAM ECC
Speed: PC2-4200E-444

Let me know if you need to know anything else about my PC & Thanks again!